Ultrasonic sensor

ABSTRACT

There are provided an ultrasonic transducer that performs transmission and/or reception of an ultrasonic wave and a propagation medium portion that forms a propagation path of the ultrasonic wave. A density ρ 1  and an acoustic velocity C 1  of the propagation medium portion and a density ρ 2  and a sound velocity C 2  of a fluid that stuffs a circumjacent space are appropriately set, and a propagation loss of the ultrasonic wave into the fluid is reduced to almost zero by refracting the ultrasonic wave at an appropriate angle.

TECHNICAL FIELD

The present invention relates to an ultrasonic sensor for transmitting or receiving an ultrasonic wave, and more precisely, to an ultrasonic transmitter for transmitting an ultrasonic wave, or an ultrasonic receiver for receiving an ultrasonic wave, or an ultrasonic transmitter-receiver for performing either one or both of them.

BACKGROUND ART

In recent years, ultrasonic transmitter-receivers are industrially utilized in a wide variety of fields of distance measurement, object detection, flow measurement, robot control and so on.

As a first ultrasonic transmitter-receiver, there is the ultrasonic transmitter-receiver described in Japanese Examined Patent Publication No. 6-101880. The construction and operation of the conventional ultrasonic transmitter-receiver will be described with reference to FIG. 10.

FIG. 10 is a sectional view of a first conventional ultrasonic transmitter-receiver. In FIG. 10, reference numeral 100 denotes an ultrasonic transmitter-receiver, 101 an ultrasonic transducer, 102 an acoustic matching layer, and 103 a housing.

In the construction of FIG. 10, operation during wave transmission will be described first. The ultrasonic transducer 101 receives a drive signal given from a drive circuit (transmitter circuit 701) via signal wires 104 and generally generates ultrasonic vibrations at a frequency in the vicinity of the resonance frequency of the ultrasonic transducer 101. The ultrasonic vibrations generated at the ultrasonic transducer 101 are transmitted to a fluid around the ultrasonic transmitter-receiver via the acoustic matching layer 102. The acoustic matching layer 103 is constructed of a material having acoustic impedance intermediate between the acoustic impedance of the circumjacent fluid and the acoustic impedance of the ultrasonic transducer 101 and has a function to improve wave transmission efficiency to the circumjacent fluid.

A piezoelectric ceramic is typically used for the ultrasonic transducer 101 that generates ultrasonic vibrations, and its acoustic impedance is, for example, about 30×10⁶ kg·m⁻²·s⁻¹. When the circumjacent fluid is a gas of air or the like, the acoustic impedance of, for example, air is about 400 kg·m⁻²·s⁻¹, the acoustic impedance of the acoustic matching layer 102 is set to about 0.11×10⁶ kg·m⁻²·s⁻¹, and the thickness is preferably set to a quarter of the wavelength at the estimated ultrasonic frequency.

Conventionally, in order to form a matching layer that has an acoustic impedance intermediate between those of the piezoelectric transducer and air, there is used a material obtained by solidifying a material (for example, glass balloons or plastic balloons) of a comparatively small density with resin.

Operation during ultrasonic wave reception will be described next. The ultrasonic wave, which has propagated through the circumjacent fluid and reached the ultrasonic transmitter-receiver 100, is transmitted to the ultrasonic transducer 101 via the acoustic matching layer 102 conversely to ultrasonic wave transmission. The ultrasonic transducer 101 converts the dynamic action of the ultrasonic wave into an electric signal, and the signal is transmitted to an electric processing section (not shown) via the signal wires 104.

During the transmission and reception operations of the ultrasonic transmitter-receiver 100 described above, the transmission and reception of an ultrasonic wave are effected in a direction in which the ultrasonic transducer 101 and the acoustic matching layer 102 are laminated, i.e., in the perpendicular direction of the acoustic matching layer 102.

As a second conventional ultrasonic transmitter-receiver, there is, for example, the ultrasonic transmitter-receiver laid open in the ultrasonic flowmeter described in Japanese Unexamined Patent Publication No. 2000-304581. The construction and operation of the conventional ultrasonic transmitter-receiver will be described below with reference to FIG. 11.

FIG. 11 is a sectional view of the second conventional ultrasonic transmitter-receiver. In FIG. 11, 104 denotes a first acoustic matching layer and 105 a second acoustic matching layer. The first acoustic matching layer 104 has a structure in which a plurality of layers of material plates (104 a, 104 b, 104 c, . . . ) differing in density and acoustic velocity are laminated and the materials are laminated in a descending order of magnitude of acoustic velocity.

Operation of the ultrasonic transmitter-receiver 100 in the construction of FIG. 11 will be described below. During wave transmission, an ultrasonic wave generated by the ultrasonic transducer 101 propagates through the first acoustic matching layer 104 (104 a, 104 b, 104 c, . . . ) and enters the second acoustic matching layer 105 by a drive signal applied from signal wires (not shown). A time during which the ultrasonic wave passes through each layer (104 a, 104 b, 104 c, . . . ) of the laminated first acoustic matching layer 104 is set so as to become equalized, and the wave front of the ultrasonic wave coincides at the interface between the first acoustic matching layer 104 and the second acoustic matching layer 105. That is, the wave propagates in the perpendicular direction at the interface to the first matching layer 104 in the second acoustic matching layer 105.

The ultrasonic wave, which has propagated through the second acoustic matching layer 105, is refracted by a difference in acoustic velocity between the second acoustic matching layer 105 and the interface of the circumjacent fluid and radiated to the circumjacent fluid with the direction thereof changed.

During wave reception, the ultrasonic wave, which has propagated through the circumjacent fluid and reached the ultrasonic transmitter-receiver 100 through the process reverse to wave transmission, is refracted at the interface to the second acoustic matching layer 105 to enter the second acoustic matching layer 105 and converted into an electric signal by the ultrasonic transducer 101 via the first acoustic matching layer 104. In this case, the acoustic wave arriving from the direction of wave transmission is selectively received.

The second conventional ultrasonic transmitter-receiver, which can integrate the ultrasonic transmitter-receiver with the wall of the measurement channel when being applied to an ultrasonic flowmeter since the direction of the acoustic wave is changed by utilizing refraction, therefore has an advantage that no disorder of the flow of the fluid to be measured is generated.

However, there has been an issue that a propagation loss has inevitably occurred and the efficiency of wave transmission and reception has been reduced even if a matching layer of a low density like the first conventional ultrasonic transmitter-receiver is used when propagating an ultrasonic wave from an ultrasonic transducer of piezoelectric ceramic or the like into a gas of air or the like. The reason why it is difficult to make an ultrasonic wave efficiently propagate from a solid to a gas is that the acoustic impedance of the gas is extremely smaller than the acoustic impedance of the solid, and a strong reflection of an ultrasonic wave disadvantageously occurs at the interface even if the matching layer is interposed.

Further, the ultrasonic transmitter-receiver of the type that effects deflection of an ultrasonic wave utilizing the refraction exhibited by the second conventional ultrasonic transmitter-receiver has had an issue that it has not substantially been applicable as a consequence of a significant reduction in the wave transmission and reception efficiency when the angle of deflection is increased due to an additionally inflicted loss caused by the angle of deflection.

Accordingly, the present invention is made in view of the aforementioned issues and has the object of providing a highly sensitive ultrasonic sensor that can deflect an ultrasonic wave and has a high efficiency of wave transmission and reception.

DISCLOSURE OF INVENTION

In order to achieve the aforementioned object, the present invention is constructed as follows.

According to the present invention, there is provided an ultrasonic sensor for performing transmission or reception of an ultrasonic wave to a circumjacent space stuffed with a fluid, the sensor comprising:

-   -   an ultrasonic transducer; and     -   a propagation medium portion that is stuffed in a space between         the ultrasonic transducer and the circumjacent space, for         forming a propagation path of the ultrasonic wave.

Further, according to the present invention, there is provided an ultrasonic sensor for performing transmission or reception of an ultrasonic wave to a circumjacent space stuffed with a fluid, the sensor comprising:

-   -   an ultrasonic transducer; and     -   a propagation medium portion that is arranged between the         ultrasonic transducer and the circumjacent space, for forming a         propagation path of the ultrasonic wave,     -   wherein a density ρ₁ of the propagation medium portion, an         acoustic velocity C₁ in the propagation medium portion, a         density ρ₂ of the fluid that stuffs the space, and a sound         velocity C₂ in the fluid that stuffs the space satisfy a         relation expressed as (ρ₂/ρ₁)<(C₁/C₂)<1.

According to the present invention, there is also provided an ultrasonic sensor for performing transmission or reception of an ultrasonic wave to a: circumjacent space stuffed with a fluid, the sensor comprising:

-   -   an ultrasonic transducer;     -   a propagation medium portion that is arranged between the         ultrasonic transducer and the circumjacent space, for forming a         propagation path of the ultrasonic wave; and     -   a reflector that is arranged in contact with the propagation         medium portion, for controlling the propagation path of the         ultrasonic wave, wherein     -   a density ρ₁ of the propagation medium portion, an acoustic         velocity C₁ in the propagation medium portion, a density ρ₂ of         the fluid that stuffs the space, and a sound velocity C₂ in the         fluid that stuffs the space satisfy a relation expressed as         (ρ₂/ρ₁)<(C₁/C₂)<1.

According to the present invention, there is provided an ultrasonic flowmeter comprising:

-   -   a flow measurement section having an inner wall that defines a         channel of a fluid to be measured;     -   at least one ultrasonic transducer that is provided outside a         channel space enclosed by the inner wall of the flow measurement         section, for performing transmission or reception of an         ultrasonic wave; and     -   a propagation medium portion that is arranged between the         ultrasonic transducer and the channel space, for forming a         propagation path of the ultrasonic wave, wherein     -   a density ρ₁ of the propagation medium portion, an acoustic         velocity C₁ in the propagation medium portion, a density ρ₂ of         the fluid to be measured, and a sound velocity C₂ of the fluid         to be measured satisfy a relation expressed as         (ρ₂/ρ₁)<(C₁/C₂)<1.

According to a 27th aspect of the present invention, there is provided an ultrasonic flowmeter comprising:

-   -   a flow measurement section having an inner wall that defines a         channel of a gas;     -   a pair of ultrasonic transducers that are provided outside a         channel space enclosed by the inner wall of the flow measurement         section, for performing transmission or reception of an         ultrasonic wave; and     -   a pair of propagation medium portions that are arranged between         each of the one pair of ultrasonic transducers and the channel         space, for refracting a propagation path of the ultrasonic wave,     -   the propagation medium portion comprising a first surface region         that faces an ultrasonic vibration surface of the ultrasonic         transducer and a second surface region that faces the channel         space,     -   the first surface region of the propagation medium portion being         inclined in a direction of flow velocity of the gas in the         channel space, and the second surface region being almost         parallel to the direction of flow velocity of the gas in the         channel space.

According to a 29th aspect of the present invention, there is provided an ultrasonic sensor for performing transmission or reception of an ultrasonic wave to a circumjacent space stuffed with a fluid, the sensor comprising:

-   -   an ultrasonic transducer; and     -   a propagation medium portion that is stuffed in a space between         the ultrasonic transducer and the circumjacent space, for         forming a propagation path of the ultrasonic wave, wherein     -   the propagation medium portion has a first surface region that         faces an ultrasonic vibration surface of the ultrasonic         transducer and a second surface region that faces a flow         stuffing the circumjacent space, and the second surface region         of the propagation medium portion is inclined with respect to         the first surface region.

BRIEF DESCRIPTION OF DRAWINGS

These and other aspects and features of the present invention will become clear from the following description taken in conjunction with the preferred embodiments thereof with reference to the accompanying drawings, in which:

FIG. 1A is a perspective general view of an ultrasonic transmitter-receiver according to a first embodiment of the present invention;

FIG. 1B is a sectional view taken along the line B-B of FIG. 1A perpendicular to the lengthwise direction of the ultrasonic transmitter-receiver of the first embodiment;

FIG. 2 is a sectional view showing the refraction of an ultrasonic wave at an interface between a propagation medium portion of the ultrasonic transmitter-receiver and a fluid in a circumjacent space thereof;

FIG. 3A is a perspective general view of an ultrasonic transmitter-receiver according to a second embodiment of the present invention;

FIG. 3B is a sectional view along a cylinder center line of the ultrasonic transmitter-receiver of the second embodiment;

FIG. 3C is a perspective view showing one example of an electrode structure of an ultrasonic transducer of the ultrasonic transmitter-receiver of the second embodiment;

FIG. 4A is a perspective general view of one ultrasonic transmitter-receiver according to a third embodiment of the present invention;

FIG. 4B is a sectional view along a cylinder center line of the ultrasonic transmitter-receiver of FIG. 4A of the third embodiment;

FIG. 5A is a perspective general view of another ultrasonic transmitter-receiver of the third embodiment of the present invention;

FIG. 5B is a sectional view along a cylinder center line of the ultrasonic transmitter-receiver of FIG. 5A of the third embodiment;

FIG. 6A is a perspective general view of an ultrasonic transmitter-receiver according to a fourth embodiment of the present invention;

FIG. 6B is a sectional view along a cylinder center line of the ultrasonic transmitter-receiver of the fourth embodiment;

FIG. 6C is a sectional view including the central axis of another cylindrical ultrasonic transmitter-receiver of the fourth embodiment;

FIG. 6D is a perspective view showing one example of the electrode surface of the ultrasonic transducer of the ultrasonic transmitter-receiver of the fourth embodiment;

FIG. 7 is a partially sectional perspective view of an ultrasonic transmitter-receiver according to a fifth embodiment of the present invention;

FIGS. 8A and 8B are general views of an ultrasonic transmitter-receiver according to a sixth embodiment of the present invention;

FIGS. 9A, 9B, and 9C are explanatory views when an ultrasonic transmitter-receiver according to a seventh embodiment of the present invention is applied to different fields;

FIG. 10 is a sectional view of a conventional ultrasonic transmitter-receiver;

FIG. 11 is a sectional view of another conventional ultrasonic transmitter-receiver;

FIGS. 12A and 12B are sectional views along the lengthwise direction of an ultrasonic flowmeter according to an eighth embodiment of the present invention and a sectional view taken along the line B-B of FIG. 12A perpendicular to the lengthwise direction of the ultrasonic flowmeter of the eighth embodiment;

FIG. 13 is a view showing the refraction of an ultrasonic wave at the interface between a propagation medium portion and a fluid to be measured;

FIG. 14 is a sectional view of an ultrasonic flowmeter according to a ninth embodiment of the present invention;

FIGS. 15A, 15B, and 15C are sectional views of an ultrasonic flowmeter according to a modification example of the eighth and ninth embodiments of the present invention;

FIGS. 16A and 16B are sectional views of an ultrasonic flowmeter according to another modification example of the eighth and ninth embodiments of the present invention; and

FIG. 17 is a sectional view of a conventional ultrasonic flowmeter.

BEST MODE FOR CARRYING OUT THE INVENTION

Before the description of the present invention proceeds, it is to be noted that like parts are designated by like reference numerals throughout the accompanying drawings.

Before describing preferred embodiments of the present invention below with reference to the drawings, various aspects of the present invention are described in advance.

According to a first aspect of the present invention, there is provided an ultrasonic sensor for performing transmission or reception of an ultrasonic wave to a circumjacent space stuffed with a fluid, the sensor comprising:

-   -   au ultrasonic transducer; and     -   a propagation medium portion that is stuffed in a space between         the ultrasonic transducer and the circumjacent space, for         forming a propagation path of the ultrasonic wave.

According to a second aspect of the present invention, there is provided an ultrasonic sensor for performing transmission or reception of an ultrasonic wave to a circumjacent space stuffed with a fluid, the sensor comprising:

-   -   an ultrasonic transducer; and     -   a propagation medium portion that is arranged between the         ultrasonic transducer and the circumjacent space, for forming a         propagation path of the ultrasonic wave,     -   wherein a density ρ₁ of the propagation medium portion, an         acoustic velocity C₁ in the propagation medium portion, a         density ρ₂ of the fluid that stuffs the space, and a sound         velocity C₂ in the fluid that stuffs the space satisfy a         relation expressed as (ρ₂/ρ₁)<(C₁/C₂)<1.

According to a third aspect of the present invention, there is provided the ultrasonic sensor as defined in the second aspect, wherein the propagation medium portion has a first surface region that faces an ultrasonic vibration surface of the ultrasonic transducer and a second surface region that faces a flow that stuffs the circumjacent space, and the second surface region of the propagation medium portion is inclined with respect to the first surface region.

According to a fourth aspect of the present invention, there is provided an ultrasonic sensor for performing transmission or reception of an ultrasonic wave to a circumjacent space stuffed with a fluid, the sensor comprising:

-   -   an ultrasonic transducer;     -   a propagation medium portion that is arranged between the         ultrasonic transducer and the circumjacent space, for forming a         propagation path of the ultrasonic wave; and     -   a reflector that is arranged in contact with the propagation         medium portion, for controlling the propagation path of the         ultrasonic wave, wherein     -   a density ρ₁ of the propagation medium portion, an acoustic         velocity C₁ in the propagation medium portion, a density ρ₂ of         the fluid that stuffs the space, and a sound velocity C₂ in the         fluid that stuffs the space satisfy a relation expressed as         (ρ₂/ρ₁)<(C₁/C₂)<1.

According to a fifth aspect of the present invention, there is provided the ultrasonic sensor as defined in the fourth aspect, wherein the propagation medium portion has a first surface region that faces an ultrasonic vibration surface of the ultrasonic transducer, a second surface region that faces a flow that stuffs the circumjacent space and at least one third surface region that is arranged between the first surface region and the second surface region in the propagation path of the ultrasonic wave and brought in contact with the reflector, and the second surface region of the propagation medium portion is inclined with respect to at least one of the first surface region and the third surface region.

According to a sixth aspect of the present invention, there is provided the ultrasonic sensor as defined in any one of the first through fifth aspects, wherein a density ρ₁ of the propagation medium portion, an incident angle θ₁ of an ultrasonic wave to an interface between the propagation medium portion and the fluid that stuffs the circumjacent space, a density ρ₂ of the fluid that stuffs the circumjacent space, and an approach angle θ₂ of the ultrasonic wave from the interface to the fluid that stuffs the circumjacent space almost satisfy a relation expressed as ρ₂/ρ₁=cotθ₂/cotθ₁.

According to a seventh aspect of the present invention, there is provided the ultrasonic sensor as defined in any one of the first through fifth aspects, wherein the propagation medium portion is formed of a dry gel of an inorganic oxide or an organic polymer.

According to an eighth aspect of the present invention, there is provided the ultrasonic sensor as defined in the sixth aspect, wherein a solid frame portion of the dry gel is made hydrophobic.

According to a ninth aspect of the present invention, there is provided the ultrasonic sensor as defined in the seventh aspect, wherein a density of the dry gel is not greater than 500 kg/m³, and a mean pore diameter of the dry gel is not greater than 100 nm.

According to a 10th aspect of the present invention, there is provided the ultrasonic sensor as defined in any one of the first through fifth aspects, comprising: an acoustic matching layer that is provided between the ultrasonic transducer and the propagation medium portion, for acoustically matching the ultrasonic transducer with the propagation medium portion.

According to an 11th aspect of the present invention, there is provided The ultrasonic sensor as defined in any one of the first through fifth aspects, wherein the fluid that stuffs the circumjacent space is a gas having a density ρ₂ of not greater than 10 kg/m³.

According to a 12th aspect of the present invention, there is provided the ultrasonic sensor as defined in any one of the first through fifth aspects, wherein a direction of transmission or reception of an ultrasonic wave is almost parallel to the second surface region.

The embodiments of the present invention will be described below.

First Embodiment

An ultrasonic transmitter-receiver as one example of the ultrasonic sensor according to the first embodiment of the present invention will be described in detail below with reference to the drawings.

The present inventor has discovered the fact that an ultrasonic wave can be propagated from a solid to a fluid (particularly, gas) causing almost no loss at an interface if the ultrasonic wave is appropriately refracted by using a propagation medium portion made of an appropriate material in an ultrasonic transmitter-receiver and then come to consider the present invention.

In the ultrasonic transmitter-receiver according to the first embodiment of the present invention, a propagation medium portion that has a plane (first surface region) parallel to the vibration surface of the ultrasonic transducer and a plane (second surface region) brought in contact with the fluid that stuffs the circumjacent space is arranged between the ultrasonic transducer and the fluid that stuffs the circumjacent space. It is to be noted that the term of “fluid that stuffs the circumjacent space” means a fluid brought in contact with at least the second surface region and does not necessarily mean a fluid that stuffs the whole periphery of the ultrasonic sensor (for example, the ultrasonic transmitter-receiver) but means a fluid that stuffs a part of the periphery within the specification and the scope of the claims of the present application.

First of all, the ultrasonic transmitter-receiver according to the first embodiment of the present invention will be described with reference to FIGS. 1A and 1B. FIG. 1A shows a perspective general view of the ultrasonic transmitter-receiver 1 of the first embodiment, and FIG. 1B shows a sectional view of the ultrasonic transmitter-receiver 1 taken along the line B-B′ of FIG. 1A.

The ultrasonic transmitter-receiver 1 shown in FIGS. 1A and 1B is provided with: an ultrasonic transducer 2 for converting electric signals into ultrasonic vibrations or converting ultrasonic vibrations into electric signals; a propagation medium portion 6 that is arranged between a fluid in a circumjacent space and the ultrasonic transducer 2 and forms a propagation path of an ultrasonic wave; an acoustic matching layer 3 that is arranged between the ultrasonic transducer 2 and the propagation medium portion 6 and provides matching of acoustic impedance between the ultrasonic transducer 2 and the propagation medium portion 6; a transducer casing 4 that houses therein the ultrasonic transducer 2 and concurrently serves as an electrical conductive path to the ultrasonic transducer 2; an insulating portion 10, arranged at a terminal plate 9 x, for preventing electrical short-circuit between two signal wires 5 for providing input and output of signals to the ultrasonic transducer 2; and a housing 9 that houses a part of the two signal wires 5, the ultrasonic transducer 2, the propagation medium portion 6, the acoustic matching layer 3, and the transducer casing 4. The housing 9 is constructed of a cylindrical side portion 9 y cut so as to incline at a prescribed angle with respect to the axial direction as shown in FIGS. 1A and 1B and the terminal plate 9 x that is fixed to a lower end portion of the side portion 9 y and functions as a part of the housing 9.

The propagation medium portion 6 is stuffed in a space that is enclosed by the housing 9 constructed of the terminal plate 9 x and the side portion 9 y, located outside the transducer casing 4 and other than the acoustic matching layer 3 arranged inside the space, and has a first surface region 7 which faces the acoustic matching layer 3 and the ultrasonic transducer 2 (note that it directly faces the acoustic matching layer 3) and a second surface region 8 which faces the fluid that stuffs the circumjacent space. Further, the second surface region 8 of the propagation medium portion 6 is inclined at a prescribed angle with respect to the first surface region 7 so as not to become parallel to the first surface region 7. In this case, the prescribed angle, as one example, which is an angle greater than 0° and smaller than 90°, preferably substantially smaller than 80°. In the first embodiment, assuming that the density of the propagation medium portion 6 is ρ₁, the acoustic velocity of the propagation medium portion 6 is C₁, the density of the fluid that stuffs the circumjacent space is ρ₂ and the acoustic velocity of the fluid that stuffs the circumjacent space is C₂, then the material of the propagation medium portion 6 is selected so as to satisfy the relation expressed by the following expression (1): (ρ₂/ρ₁)<(C ₁ /C ₂)<1  (1)

When the fluid is a gas of air or the like, it is difficult to find a material that satisfies the above-mentioned condition. The reason for the above is that there is few solid materials whose acoustic velocity C₁ is smaller than the acoustic velocity C₂ of gas. In the first embodiment, in order to provide the propagation medium portion 6 that satisfies the aforementioned condition, the propagation medium portion 6 is formed of a dry gel of an inorganic oxide or an organic polymer. The solid frame portion of the dry gel employed in the first embodiment is made hydrophobic, and the density thereof is not greater than 500 kg/m³. This dry gel is a nano-porous dry gel (nanoporous dry gel) having a mean pore diameter of not greater than 100 nm.

The solid frame portion of the dry gel of the inorganic oxide preferably has an ingredient of at least silicon oxide (silica) or aluminum oxide (alumina). Moreover, the solid frame portion of the dry gel of the organic polymer can be constructed of a general thermosetting resin or a thermoplastic resin. For example, there can be used polyurethane, polyurea, phenol cured resin, polyacrylamide, polymethyl methacrylate, or the like.

In the case where the propagation medium portion 6 is formed of a nanoporous dry gel that has a main ingredient of, for example, silica, if the density ρ₁ is 200 kg/m³, then the acoustic velocity C₁ can be set within a range of about 100 m/s to 180 m/s. When the fluid that stuffs the circumjacent space is air, since the density ρ₂ of air is 1.22 kg/m³ and the acoustic velocity C₂ is 340 m/s, it is possible to concurrently satisfy the relations expressed as ρ₂<ρ₁ and C₁<C₂ and satisfy the relation expressed as (ρ₂/ρ₁)<(C₁/C₂) by adopting the above propagation medium portion 6. When measuring a gas such as natural gas, the propagation medium portion 6 preferably has a density ρ₁ ranging from 100 to 300 kg/m³ and an acoustic velocity C₁ ranging from 100 to 300 m/s.

The ultrasonic transducer 2 is a piezoelectric element and is able to generate an electric signal by the generation of ultrasonic vibrations and/or the arrival of an ultrasonic wave as a consequence of applying an electric signal. Piezoelectric ceramics are suitably employed as the piezoelectric material. If it is desired to control the resonance characteristic and reduce the mechanical Q-value, an absorber may be peripherally arranged.

The ultrasonic transmitter-receiver 1 of the first embodiment has a function to improve the acoustic matching between the ultrasonic transducer 2 that is the ultrasonic generating source and the propagation medium portion 6 by providing the acoustic matching layer 3 between the propagation medium portion 6 and the ultrasonic transducer 2.

In the case where the propagation medium portion 6 is formed of the nanoporous dry gel (acoustic impedance: 3×10⁴ kg·m⁻²·s⁻¹) having a main ingredient of silica and the ultrasonic transducer 1 is constructed of a piezoelectric ceramic (acoustic impedance: 30×10⁶ kg·m·m⁻²·s⁻¹), by adopting an acoustic matching layer 3 produced from a material having an acoustic impedance in the vicinity of 1×10⁶ kg·m⁻²·s⁻¹, the propagation efficiency of ultrasonic energy can be made almost one or concretely not smaller than 0.95. The above material can be provided by a composite material obtained by solidifying a hollow glass ball with a resin material, or a porous ceramic. The thickness of the acoustic matching layer 3 is preferably set to a quarter wavelength of the ultrasonic wave used.

Furthermore, in the ultrasonic transmitter-receiver 1 of the first embodiment, the ultrasonic transducer 2 is housed in the transducer casing 4, while the ultrasonic transducer 2 is bonded to the inside of the top surface of the transducer casing 4 and the acoustic matching layer 3 is bonded to the outside of the top surface of the transducer casing 4, constituting a laminate structure. The transducer casing 4 is preferably formed of a conductive metallic material of stainless steel or the like, and the acoustic matching between the ultrasonic transducer 2 and the acoustic matching layer 3 can be maintained in a satisfactory state by thickness setting to a thickness of not greater than {fraction (1/10)} or preferably not greater than {fraction (1/20)} of the wavelength of the estimated ultrasonic wave.

The transducer casing 4 is bonded to the terminal plate 9 x of the housing 9 by a processing method of electric welding or the like and is able to have a hermetic structure stuffed with an inert gas of dry nitrogen, argon, or the like. With the above-mentioned arrangement, the ultrasonic transducer 2 is physically insulated from the external air environment, allowing the reliability to be improved. In addition, there is provided an electrically shielded structure, and therefore, high safety can be secured even when the fluid that stuffs the circumjacent space is a flammable fluid of natural gas or the like.

Next, the behavior of an ultrasonic wave propagating from the propagation medium portion 6 to the fluid that stuffs the circumjacent space will be described in detail below with reference to FIG. 2.

According to the aforementioned relation of arrangement, the ultrasonic wave enters from the normal direction of the first surface region 7 that faces the vibration surface of the ultrasonic transmitter-receiver 1 and is parallel to the vibration surface. Therefore, the ultrasonic wave enters along a direction inclined with respect to the normal direction of the second surface region 8 that is the interface between the propagation medium portion 6 and the fluid that stuffs the circumjacent space. It is assumed that the incident angle of the ultrasonic wave with respect to the normal direction of the second surface region 8 is θ₁ (0°<θ₁<90°). At this time, the ultrasonic wave is refracted at the second surface region 8 that is the interface between the propagation medium portion 6 and the fluid that stuffs the circumjacent space and enters the fluid to be measured at an angle θ₂ (approach angle) with respect to the normal direction (θ₁<θ₂).

In the first embodiment, the various parameters (ρ₁, θ₁, and θ₂) are set so as to almost satisfy the relation of the following expression (2) when the density ρ₂ of the fluid that stuffs the circumjacent space is given. (ρ₂/ρ₁)=(cotθ₂/cotθ₁)  (2)

With the above setting, propagation efficiency from the propagation medium portion 6 of ultrasonic energy to the fluid that stuffs the circumjacent space becomes almost one. At this time, the incident angle θ₁ satisfies the condition expressed by the following expression (3). (cotθ₁)²=[(c₁/c₂)²−1]/[(ρ₂/ρ₁)²−(c₁/c₂)²]  (3)

Therefore, if ρ₁ and C₁ of the propagation medium portion 6 and ρ₂ and C₂ of the fluid that stuffs the circumjacent space are determined, then the incident angle θ₁ is determined according to the expression (3). Moreover, if the incident angle θ₁ is determined, then the approach angle θ₂ is also determined according to the expression (2). If the incident angle θ₁ and the approach angle θ₂ are determined, then the inclination angle and so on of the second surface region 8 of the propagation medium portion 6 can also be determined.

The above-mentioned fact is also applied to the case where the ultrasonic wave, which has propagated through the fluid that stuffs the circumjacent space, is received, and therefore, the ultrasonic wave arriving from the direction of the approach angle θ₂ is selectively received.

In the first embodiment, by forming the propagation medium portion 6 of the aforementioned material, the acoustic velocity C₁ of the propagation medium portion 6 can be set to 180 m/s, and the density ρ₁ can be set to 200 kg/m³. In the case where the circumjacent space is an ordinary space stuffed with air, the density ρ₂ of air is 1.22 kg/m³ and the acoustic velocity C₂ is 340 m/s. Therefore, according to the relations of the expression (2) and the expression (3), it is proper to set the incident angle θ₁ to 32° and set the approach angle θ₂ to 89°. Since the approach angle θ₂ is close to 90°, the ultrasonic wave transmitted in the air travels almost parallel to the second surface region 8 that is the wave transmission surface. Therefore, the direction of transmitting and receiving an acoustic wave in the first embodiment is directed in the direction of arrow 90 of FIG. 1A along the line segment B-B′ in a plane that includes the second surface region 8 shown in FIG. 1A.

According to the first embodiment, almost no propagation loss occurs in the second surface region 8 that is the interface between the propagation medium portion 6 and the fluid that stuffs the circumjacent space, and therefore, it is not required to match their acoustic impedances at this interface with each other. Therefore, the ultrasonic wave emitted from inside the propagation medium portion 6 is refracted at the second surface region 8 that is the interface between the propagation medium portion 6 and the fluid that stuffs the circumjacent space, allowing the ultrasonic wave to deflect in the direction along the plane that includes the second surface region 8. In addition, almost no propagation loss occurs in the second surface region 8, and therefore, a highly sensitive ultrasonic sensor that has high wave transmitting and receiving efficiency can be provided.

It is to be noted that the propagation medium portion 6 is not required to be constructed of a material whose density ρ₁ and acoustic velocity C₁ are uniform throughout the entire body but allowed to have a laminate structure in which a plurality of kinds of material layers having varied densities ρ₁ and acoustic velocities C₁ are laminated. When the laminate structure as described above is possessed, it is sometimes the case where the ultrasonic wave does not straightly travel through the propagation medium portion 6, which, however, poses no problem. An important point is that the density ρ₁ and the acoustic velocity C₁ and the incident angle θ₁ of the propagation medium portion 6 are set so as to satisfy the aforementioned expressions in the region in the vicinity of the interface between the propagation medium portion 6 and the fluid to be measured.

Operation of the ultrasonic transmitter-receiver of the first embodiment will be described next.

First of all, when transmitting wave to the fluid that stuffs the circumjacent space, an ac voltage, a pulse voltage, or a burst voltage having a frequency in the vicinity of the resonance frequency (for example, about 100 kHz to 1 MHz) is applied from a transmitter circuit 701 that concurrently serves as a drive circuit shown in FIG. 9C via the signal wires 5 to the ultrasonic transducer 2 (81 in FIG. 9C). By this operation, vibrations in the vicinity of the resonance frequency are excited in the ultrasonic transducer 2, and thus, the vibrations are radiated as an ultrasonic wave on the condition of an efficiency of almost one to the propagation medium portion 6 through the transducer casing 4 and the acoustic matching layer 3. The acoustic wave, which has propagated through the propagation medium portion 6, is refracted at the second surface region 8 that is the interface between the propagation medium portion 6 and the fluid that stuffs the circumjacent space and radiated to the fluid that stuffs the circumjacent space in the state of an efficiency of almost one.

Next, when receiving the ultrasonic wave, which propagates through the fluid that stuffs the circumjacent space and reaches the ultrasonic transmitter-receiver 1, a path reverse to that of wave transmission holds with regard to the ultrasonic wave that have propagated from the wave transmission direction. The ultrasonic wave, which have entered the transmitter-receiver 1, reaches the ultrasonic transducer 2 on the condition of an efficiency of almost one and are converted into electric signals and transmitted to an external electric circuit (for example, a receiver circuit 702) through the signal wires 5.

According to the first embodiment, there is provided the propagation medium portion 6 that exhibits the appropriate density ρ₁ and acoustic velocity C₁, and the ultrasonic wave is refracted at an appropriate angle. Therefore, the propagation loss at the interface between the substances is made almost zero, allowing the flow measurement to be achieved at a satisfactory signal-to-noise ratio. Then, according to the first embodiment, the transmission and reception of an ultrasonic wave through a gas (for example, hydrogen gas or the like), which has had extreme difficulties in transmitting and receiving an ultrasonic wave in the conventional ultrasonic transmitter-receiver, becomes possible scarcely generating loss at the interface by appropriately refracting the ultrasonic wave by means of the propagation medium portion 6, and therefore, an application to the flow measurement capable of measuring these gases becomes possible.

Furthermore, in the ultrasonic transmitter-receiver 1 of the first embodiment, the ultrasonic wave emitted from the propagation medium portion 6 is refracted at the second surface region 8 that is the interface between the propagation medium portion 6 and the fluid that stuffs the circumjacent space, and the direction in which the ultrasonic wave are transmitted and received is deflected in the direction along the plane that includes the second surface region 8. Therefore, in, for example, a flowmeter, projections and recesses concerning the mounting of the transmitter-receiver in the measurement channel are removed, and a flowmeter free from the disorder of fluid flow can be constructed. Moreover, the flowmeter can also be applied to object detection in the horizontal direction and so on even when the flowmeter is mounted while being aligned with the horizontal plane of a variety of kinds of equipment that has a horizontal portion.

Second Embodiment

An ultrasonic transmitter-receiver as one example of the ultrasonic sensor according to the second embodiment of the present invention will be described with reference to FIGS. 3A, 3B and 3C. FIG. 3A is a perspective view of an appearance of an ultrasonic transmitter-receiver 11 of the second embodiment. FIG. 3B is a sectional view including the central axis of the cylindrical ultrasonic transmitter-receiver 11. FIG. 3C is a perspective view showing one example of the electrode structure of an ultrasonic transducer 12. Like reference numerals are given to the common members of the second embodiment and the first embodiment. A propagation medium portion 6A corresponds to the propagation medium portion 6 of the first embodiment, and a housing 9A corresponds to the housing 9 of the first embodiment.

Hereinafter, characteristic points of the ultrasonic transmitter-receiver 11 of the second embodiment will be described below, and no description is provided for portions similar to those of the ultrasonic transmitter-receiver 1 of the first embodiment and the aforementioned corresponding portions.

In the ultrasonic transmitter-receiver 11 of the second embodiment, a cylindrical side portion 9 z is fixed to a disk-shaped terminal plate 9 x constituting the housing 9A, providing a construction symmetrical to the axis around the center shaft 9 a fixed to the center of the terminal plate 9 x. Therefore, an ultrasonic transducer 12 and an acoustic matching layer 13 arranged on the disk-shaped terminal plate 9 x of the housing 9A are constructed in a ring-like shape. Moreover, a disk-shaped protecting section 15, which is part of the housing 9A and of which the center portion is connected to the center shaft 9 a, is further provided to protect the second surface region 8. A ring-shaped opening 14 is provided between the housing 9A and the protecting section 15, and an ultrasonic wave is transmitted and received in the direction of arrow 90A through the opening 14. A space, which is located inside the housing 9A and is other than the ultrasonic transducer 12 and the acoustic matching layer 13, is almost stuffed with the propagation medium portion 6A. Then, the propagation medium portion 6A has a first surface region 7 that faces the acoustic matching layer 13 and the ultrasonic transducer 12 (note that the first surface region 7 directly faces the acoustic matching layer 13) and a second surface region 8 that faces the fluid that stuffs the circumjacent space. Further, the second surface region 8 of the propagation medium portion 6A is approximately uniformly inclined at a prescribed angle from the periphery toward the center side with respect to the center shaft 9 a so that the second surface region 8 does not become parallel to the first surface region 7.

In the second embodiment, the concrete transmission and reception of an ultrasonic wave are performed with high efficiency similarly to the first embodiment, so that effects similar to those of the first embodiment can be produced. A difference from the first embodiment resides in that omnidirectional wave transmission and reception around the center shaft 9 a can be achieved since the ultrasonic transmitter-receiver 11 has the structure symmetrical to the center shaft 9 a. When the circumjacent space is stuffed with a gas, the wave transmission and reception become almost horizontal, and an application to omnidirectional object sensing and so on becomes possible.

FIG. 3C shows one example of the electrode structure of the ultrasonic transducer 12, and the reference numeral 16 denotes segmented electrode sections constructed on the surface of the electrode. By the electrode sections 16 segmented as shown in FIG. 3C, the vibration generating section can be controlled. Therefore, scanning by an ultrasonic wave in the circumferential direction becomes possible, and an application to object sensing and so on with the direction specified becomes possible.

The segmentation of the electrode sections is allowed to be achieved on at least one surface of the electrode sections 16 formed on the front and rear sides. Moreover, a similar effect can be obtained also by arranging the ultrasonic transducer 12 while segmenting the transducer 12 itself. Although the number of segmentation of the electrode section 6 is four in FIG. 3C, the number of segmentation is arbitrary, and it is not necessarily required to provide an identical shape or a symmetrical shape.

Third Embodiment

An ultrasonic transmitter-receiver as one example of the ultrasonic sensor according to the third embodiment of the present invention will be described with reference to FIGS. 4A and 4B and FIGS. 5A and 5B. FIG. 4A shows a perspective view of the external appearance of an ultrasonic transmitter-receiver 21 of the third embodiment. FIG. 4B shows a cross section of the cylindrical ultrasonic transmitter-receiver 21 including its central axis. FIG. 5A shows a perspective view of the external appearance of another ultrasonic transmitter-receiver 31 of the third embodiment. FIG. 5B shows a cross section of the cylindrical ultrasonic transmitter-receiver 31 including its center shaft 9 b. Like reference numerals are given to the common members of the third embodiment and the aforementioned first and second embodiments. It is to be noted that propagation medium portions 6B and 6C correspond to the propagation medium portion 6 or 6A of the foregoing embodiments, and housings 9B and 9C correspond to the housing 9 or 9A of the foregoing embodiments.

Hereinafter, characteristic points of the ultrasonic transmitter-receiver 21 and the ultrasonic transmitter-receiver 31 of the third embodiment will be described below, and no description is provided for portions similar to those of the ultrasonic transmitter-receiver 1 of the first embodiment and the ultrasonic transmitter-receiver 11 of the second embodiment and the aforementioned corresponding portions.

In the housing 9B of the ultrasonic transmitter-receiver 21 of the third embodiment, as shown in FIGS. 4A and 4B, a terminal plate 9 x is fixed to the lower end of a cylindrical side portion 9 g, and a disk-shaped upper plate 9 h having a circular opening 24 in a center portion is fixed to the upper end of the side portion 9 g, so that the housing 9B is constructed symmetrically to the axis around the imaginary central axis. An ultrasonic transducer 22 and an acoustic matching layer 23 fixed to the inner surface of the side portion 9 g of the housing 9B are cylindrically constructed. Moreover, the propagation medium portion 6B is housed and arranged in the housing 9B so that the propagation medium portion 6B does not protrude inwardly of the center opening 24 of the disk-shaped upper plate 9 b of the housing 9B to protect the second surface region 8 by the entire body of the housing 9B, and the transmission and reception of an ultrasonic wave are performed in the direction of arrow 90B through the center opening 24 of the upper plate 9 b of the housing 9B. That is, a space, which is located inside the housing 9B and is other than the ultrasonic transducer 22, the acoustic matching layer 23 and the center portion, is stuffed with the propagation medium portion 6B. Then, the propagation medium portion 6B has a first surface region 7 that faces the acoustic matching layer 23 and the ultrasonic transducer 22 (note that the first surface region 7 directly faces the acoustic matching layer 23) and a second surface region 8 that faces the fluid that stuffs the circumjacent space (the second surface region 8 faces the space in the center portion in FIG. 4B). Further, the second surface region 8 of the propagation medium portion 6B is inclined at a prescribed angle forming a conical surface so that the second surface region 8 expands from the upper end to the lower end roughly uniformly with respect to the central axis and does not become parallel to the first surface region 7.

On the other hand, as shown in FIGS. 5A and 5B, in the housing 9C of another ultrasonic transmitter-receiver 31 of the third embodiment, a terminal plate 9 x is fixed to the lower end of a cylindrical side portion 35, and a center portion of a disk-shaped upper plate 9 i is fixed to the center shaft 9 b fixed to the center portion of the terminal plate 9 x, so that the housing 9C is constructed symmetrically to the axis around the center shaft 9 b. An ultrasonic transducer 32 and an acoustic matching layer 33 fixed around the center shaft 9 b are cylindrically constructed. Moreover, a propagation medium portion 6C is housed and arranged in the housing 9C so that the propagation medium portion 6C does not protrude outwardly of the periphery of an upper plate 9 i of the housing 9C to protect the second surface region 8 by a protecting portion 35 that is the cylindrical side portion fixed to the terminal plate 9 x of the housing 9C. A ring-shaped opening 34 is provided between the housing 9C and the protecting portion 35, and the transmission and reception of an ultrasonic wave are performed in the direction of arrow 90C through the opening 34. That is, a space, which is located inside the housing 9C and is other than the ultrasonic transducer 32, the acoustic matching layer 33, and the peripheral portion, is stuffed with the propagation medium portion 6C. Then, the propagation medium portion 6C has a first surface region 7 that faces the acoustic matching layer 33 and the ultrasonic transducer 32 (note that the first surface region 7 directly faces the acoustic matching layer 33) and a second surface region 8 that faces the fluid that stuffs the circumjacent space (the second surface region 8 faces the space in the peripheral portion in FIG. 5B). Further, the second surface region 8 of the propagation medium portion 6C is inclined at a prescribed angle forming a conical surface so that the second surface region 8 expands from the lower end to the upper end roughly uniformly with respect to the central axis and does not become parallel to the first surface region 7.

In the third embodiment, the concrete transmission and reception of an ultrasonic wave are performed with high efficiency similarly to the first embodiment and the second embodiment, and effects similar to those of the first embodiment and the second embodiment can be produced. A difference from the second embodiment resides in that the transmission and reception of an ultrasonic wave are performed in the forward direction (upward direction in FIGS. 4B and 5B) of the ultrasonic transmitter-receivers 21 and 31, and an application to the generally materialized ultrasonic transmitter-receivers is possible.

Fourth Embodiment

An ultrasonic transmitter-receiver as one example of the ultrasonic sensor according to the fourth embodiment of the present invention will be described with reference to FIGS. 6A, 6B, 6C and 6D. FIG. 6A shows a perspective view of the external appearance of an ultrasonic transmitter-receiver 41 and an ultrasonic transmitter-receiver 51 of the fourth embodiment. FIG. 6B is a sectional view of the cylindrical ultrasonic transmitter-receiver 41 including its center shaft. FIG. 6C is a sectional view of another cylindrical ultrasonic transmitter-receiver 51 of the fourth embodiment including its center shaft. FIG. 6D shows a perspective view of one example of the electrode surfaces of ultrasonic transducers 42 and 52 of the ultrasonic transmitter-receiver of the fourth embodiment. Like reference numerals are given to the common members of the fourth embodiment and the first through third embodiments. It is to be noted that propagation medium portions 6D and 6E correspond to the propagation medium portion 6, 6A or the like of the foregoing embodiments. Housings 9D and 9E correspond to the housing 9, 9A or the like of the foregoing embodiments.

Hereinafter, characteristic points of the ultrasonic transmitter-receiver 41 and the ultrasonic transmitter-receiver 51 of the fourth embodiment will be described below, and no description is provided for portions similar to those of the ultrasonic transmitter-receiver 1 of the first embodiment and the ultrasonic transmitter-receivers 11 and 21 of the second embodiment and the aforementioned corresponding portions.

In the housing 9D of the ultrasonic transmitter-receiver 41 of the fourth embodiment, as shown in FIGS. 6A and 6B, a terminal plate 9 x is fixed to the lower end of a cylindrical side portion 9 g, and a truncated-circular-cone-shaped reflector 44 is fixed to a center portion of the terminal plate 9 x, so that the housing 9D is constructed symmetrically to the axis around the central axis of the reflector 44. An ultrasonic transducer 42 and an acoustic matching layer 43 fixed to a side portion 9 g of the housing 9D and the inner surface of the terminal plate 9 x are cylindrical, and the acoustic matching layer 43 is arranged inside the ultrasonic transducer 42. Moreover, the propagation medium portion 6D is housed and arranged in the housing 9D so that the propagation medium portion 6D does not protrude from the upper end surface of the reflector 44 of the housing 9D and the upper end surface of the side portion 9 g. Then, the propagation medium portion 6D has a first surface region 7 parallel to the vibration surface of the ultrasonic transducer 42 and a second surface region 8 brought in contact with the fluid that stuffs the circumjacent space and is brought in contact with the reflector 44 provided adjacent to the propagation medium portion 6D by a third surface region 45.

The reflector 44 is constructed of a metallic material of stainless steel or the like, and, when the propagation medium portion 6 is formed of the nanoporous dry gel that has a main ingredient of, for example, silica, the reflection efficiency in the third surface region 45 becomes almost one. With regard to the reflector 44, an inclination angle is set with respect to the first surface region 7 and the second surface region 8 so that the incident angle of an ultrasonic wave to the second surface region 8 satisfies the expression (3).

The ultrasonic transducer 42 is excited with vibrations in the vicinity of the resonance frequency, and the vibrations are radiated as an ultrasonic wave on the condition of an efficiency of almost one roughly on the center side of the propagation medium portion 6D through the acoustic matching layer 43. The acoustic wave, which have propagated through the propagation medium portion 6D, is reflected on the third surface region 45 that is the interface to the reflector 44 with an efficiency of almost one to propagate with the direction thereof changed roughly toward the second surface region 8 side, then refracted at the second surface region 8 that is the interface between the propagation medium portion 6D and the fluid that stuffs the circumjacent space, and radiated to the fluid that stuffs the circumjacent space with an efficiency of almost one.

Moreover, when receiving the ultrasonic wave, which propagates through the fluid that stuffs the circumjacent space and reach the ultrasonic transmitter-receiver 41, a path reverse to that of wave transmission holds with regard to the ultrasonic wave that has propagated from the direction of wave transmission. The ultrasonic wave, which enters the second surface region 8 of the ultrasonic transmitter-receiver 41 with an efficiency of almost one, propagates through the propagation medium portion 6D and are reflected with an efficiency of almost one on the third surface region 45 that is the interface to the reflector 44 to propagate with the direction thereof changed roughly toward the ultrasonic transducer 42 side and reach the ultrasonic transducer 42 and then converted into electric signals by the ultrasonic transducer 42.

With the above construction, the ultrasonic transmitter-receiver 41 also becomes able to highly efficiently transmit and receive an ultrasonic wave to and from the fluid that stuffs the circumjacent space, and effects similar to those of the first embodiment can be produced.

On the other hand, as shown in FIGS. 6A and 6C, in the housing 9E of another ultrasonic transmitter-receiver 51 of the fourth embodiment, a terminal plate 9 x is fixed to the lower end of a sectionally triangular cylindrical reflector 54, a columnar center shaft 9 k is fixed to a center portion of the terminal plate 9 x, and a disk-shaped upper plate 9 m is fixed like a protruded flange to the upper end of the center shaft 9 k, so that the housing 9E is constructed symmetrically to the axis around the central axis of the reflector 54. An ultrasonic transducer 52 and an acoustic matching layer 53 fixed to the center shaft 9 k of the housing 9E and the inner surfaces of the upper plate 9 m and the terminal plate 9 x are cylindrical, and the acoustic matching layer 53 is arranged outside the ultrasonic transducer 52. Moreover, the propagation medium portion 6E is housed and arranged in the housing 9E so that the propagation medium portion 6E does not protrude above the upper end surface of the reflector 54 of the housing 9E and the upper end surface of the upper plate 9 m. Then, the propagation medium portion 6E has a first surface region 7 parallel to the vibration surface of the ultrasonic transducer 52 and a second surface region 8 brought in contact with the fluid that stuffs the circumjacent space, and is brought in contact with the reflector 54 provided adjacent to the propagation medium portion 6E by a third surface region 55. The reflector 54 has a material and an inclination angle similar to those of the reflector 44.

The ultrasonic transducer 52 is excited with vibrations in the vicinity of the resonance frequency, and the vibrations are radiated as an ultrasonic wave roughly to the peripheral side of the propagation medium portion 6E through the acoustic matching layer 53 on the condition of an efficiency of almost one. The ultrasonic wave, which has propagated through the propagation medium portion 6E, is reflected on the third surface region 55 that is the interface to the reflector 54 with an efficiency of almost one to propagate with the direction thereof changed roughly toward the second surface region 8 side, refracted at the second surface region 8 that is the interface between the propagation medium portion 6E and the fluid that stuffs the circumjacent space and then radiated to the fluid that stuffs the circumjacent space with an efficiency of almost one.

Moreover, when receiving the ultrasonic wave, which propagates through the fluid that stuffs the circumjacent space and reaches the ultrasonic transmitter-receiver 51, a path reverse to that of wave transmission holds with regard to the ultrasonic wave that has propagated from the wave transmission direction. The ultrasonic wave, which enters the second surface region 8 of the ultrasonic transmitter-receiver 51 with an efficiency of almost one, propagates through the propagation medium portion 6E and is reflected at the third surface region 55 that is the interface to the reflector 54 with an efficiency of almost one to propagate with the direction thereof changed roughly toward the ultrasonic transducer 52 side and reach the ultrasonic transducer 52, and then converted into electric signals by the ultrasonic transducer 52.

With the above construction, the ultrasonic transmitter-receiver 51 also becomes able to highly efficiently transmit and receive an ultrasonic wave to and from the fluid that stuffs the circumjacent space, and effects similar to those of the first embodiment can be produced.

It is to be noted that a plurality of reflectors 44 and 54 of the fourth embodiment may be arranged, and in the above case, there is existing a plurality of third surface regions 45 and 55 in terms of construction. Moreover, in this case, a construction in which the first surface region 7 and the second surface region 8 are parallel to each other also holds, and at least one of the plurality of third surface regions 45 and 55 is required to be inclined at a prescribed angle with respect to the second surface region 8.

The ultrasonic transmitter-receiver 41 and the ultrasonic transmitter-receiver 51 have a structure in which wave transmission and reception can be performed omnidirectionally around the central axis similarly to the second embodiment. When the circumjacent space is stuffed with a gas, the wave transmission and reception become almost horizontal, and an application to omnidirectional object sensing and so on becomes possible.

Moreover, FIG. 6D shows one example of the electrode structure of the ultrasonic transducers 42 and 52. The reference numeral 46 denotes segmented electrodes constructed on the inner side surfaces of the cylindrical ultrasonic transducers 42 and 52, and the reference numeral 47 denotes a common electrode constructed on the outer side surface. By the electrode sections 46 segmented as shown in FIG. 6D, the vibration generating section can be controlled. Therefore, scanning by an ultrasonic wave in the circumferential direction becomes possible, and an application to object sensing and so on with the direction specified becomes possible.

As shown in FIG. 6D, the segmentation of the electrode sections 46 is allowed to be achieved on at least one side surface of the electrode sections formed on the inner and outer side surfaces. Moreover, a similar effect can be obtained also by arranging the ultrasonic transducers 42 and 52 while segmenting the transducers 42 and 45 themselves. The number of segmentation of the electrode is arbitrary, and it is not necessarily required to provide an identical shape or a symmetrical shape.

Fifth Embodiment

The ultrasonic transmitter-receiver as one example of the ultrasonic sensor according to the fifth embodiment of the present invention will be described with reference to FIG. 7. FIG. 7 is a perspective view of the external appearance of the ultrasonic transmitter-receiver 61 of the fifth embodiment, also serving as a sectional view for explaining the internal structure with regard to the plane A on this side of FIG. 7, the plane actually covered with a housing side portion. Moreover, like reference numerals are given to the common members of the fifth embodiment and the first through fourth embodiments. It is to be noted that propagation medium portions 6F correspond to the propagation medium portions 6, 6A or the like of the first embodiment, and a housing 9F corresponds to the housing 9, 9A or the like of the first embodiment.

Hereinafter, characteristic points of the ultrasonic transmitter-receiver 61 of the fifth embodiment will be described below, and no description is provided for portions similar to those of the ultrasonic transmitter-receiver 1 of the first embodiment, the ultrasonic transmitter-receivers 11 and 21 of the second embodiment, the ultrasonic transmitter-receiver 31 of the third embodiment, the ultrasonic transmitter-receivers 41 and 51 of the fourth embodiment and the aforementioned corresponding portions.

In the ultrasonic transmitter-receiver 61 of the fifth embodiment, the portion, which are the cylindrical or conical portion of the ultrasonic transmitter-receivers 41 and 51 of the fourth embodiment are made rectangular. A rectangular plate-shaped terminal plate 69 x is fixed to the lower end of a rectangular plate-shaped side portion 69 g, a triangular prismatic reflector 64 is fixed to the peripheral portions of the side portion 69 g and the terminal plate 69 x, and a rectangular disk-shaped upper plate 69 m is fixed, with protruded like a flange, to the upper end of a rectangular plate-shaped center shaft 69 k, providing a symmetrical construction with respect to a center line C. Ultrasonic transducers 62 and acoustic matching layers 63 are rectangular plates and are fixed to the inner surface of the center shaft 69 k, and the acoustic matching layers 63 are arranged outside the ultrasonic transducers 62. The propagation medium portions 6F have a first surface region 7 parallel to the vibration surface of the ultrasonic transducer 62 and a second surface region 8 brought in contact with the fluid that stuffs the circumjacent space and is brought in contact with reflectors 64 provided adjacent to the propagation medium portions 6F by third surface regions 65. The reflectors 64 have a material and an inclination angle which are similar to those of the reflector 44.

The fifth embodiment differs from the fourth embodiment in that the transmission and reception of an ultrasonic wave in the ultrasonic transmitter-receiver 61 of the fifth embodiment are performed transversely in FIG. 7 and symmetrically with respect to the center line C. Even in this case, ultrasonic waves are radiated to the propagation medium portions 6F through the respective acoustic matching layers 63 on the condition of an efficiency of almost one, and the ultrasonic waves, which have propagated through the propagation medium portion 6F, are reflected at the third surface region 65 that is the interface to reflector 64 with an efficiency of almost one to change the direction roughly toward the second surface region 8 side. Further, the ultrasonic waves are refracted at the second surface regions 8 that are the interfaces between the propagation medium portions 6F and the fluid that stuffs the circumjacent spaces and radiated to the fluid that stuffs the circumjacent space, with an efficiency of almost one. Moreover, a path reverse to that of wave transmission holds with regard to the ultrasonic wave that has propagated from the direction of wave transmission. The ultrasonic waves, which have entered the second surface regions 8 of the ultrasonic transmitter-receiver 61 with an efficiency of almost one, propagate through the propagation medium portions 6F and are reflected at the third surface regions 65 that are the interfaces to reflectors 64 with an efficiency of almost one to reach the ultrasonic transducers 62, and then converted into electric signals by the ultrasonic transducers 62.

With the above construction, the ultrasonic transmitter-receiver 61 also becomes able to highly efficiently transmit and receive ultrasonic waves to and from the fluid that stuffs the circumjacent space, and effects similar to those of the first embodiment can be produced. Moreover, the ultrasonic transmitter-receiver 61 has a structure such that wave transmission and reception in the transverse direction can be performed, and when the circumjacent space is stuffed with a gas, the transmission and reception waves become almost horizontal, and an application to transverse object sensing and so on becomes possible.

Sixth Embodiment

An ultrasonic transmitter-receiver as one example of the ultrasonic sensor according to the sixth embodiment of the present invention will be described with reference to FIGS. 8A and 8B. FIGS. 8A and 8B are perspective views of a plurality of arranged ultrasonic transmitter-receivers 71-1, 71-2, 71-3 and 71-4 of the sixth embodiment, and like reference numerals are given to common members of the sixth embodiment and the first through fifth embodiments. It is to be noted that a propagation medium portion 6G corresponds to the propagation medium portion 6 of the first embodiment, the propagation medium portions 6F of the fifth embodiment and so on, and a housing 9G corresponds to the housing 9 of the first embodiment, the housing 9F of the fifth embodiment and so on. A center shaft 79 k corresponds to the center plate 69 k of the fifth embodiment, and a side portion 79 g corresponds to the side portions 69 g of the fifth embodiment. A terminal plate 79 x corresponds to the terminal plate 69 x of the fifth embodiment, a reflector 74 corresponds to the reflectors 64 of the fifth embodiment, and an upper plate 79 m corresponds to the upper plates 69 m of the fifth embodiment.

Hereinafter, characteristic points of the ultrasonic transmitter-receivers 71-1, 71-2, 71-3 and 71-4 of the sixth embodiment will be described below, and no description is provided for portions similar to those of other embodiments and the aforementioned corresponding portions.

The ultrasonic transmitter-receivers 71-1, 71-2, 71-3 and 71-4 of the sixth embodiment have a structure in which the ultrasonic transmitter-receiver 61 of the fifth embodiment is divided along the center line C. Therefore, although the transmission and reception of an ultrasonic wave are limited to one direction, the transmission and reception of an ultrasonic wave are performed with high efficiency similarly to the fifth embodiment, and effects similar to those of the first embodiment can be produced. When the circumjacent space is stuffed with a gas, the wave transmission and reception becomes almost horizontal, and an application to object sensing and so on becomes possible.

FIG. 8A shows a construction in which the four ultrasonic transmitter-receivers 71-1, 71-2, 71-3 and 71-4 are employed to allow ultrasonic waves to be transmitted and received in the depthwise and transverse directions, and FIG. 8B shows a construction in which three ultrasonic transmitter-receivers 71-1, 71-2 and 71-3 are employed to allow ultrasonic waves to be transmitted and received in three directions at intervals of 120 degrees. As shown in FIGS. 8A and 8B, by arranging an arbitrary plural number of ultrasonic transmitter-receivers 71 in arbitrary directions, control of the directivity corresponding to the application becomes possible, and a range of application can be widened.

Seventh Embodiment

Application equipment of an ultrasonic transmitter-receiver as one example of the ultrasonic sensor according to the seventh embodiment of the present invention will be described with reference to FIGS. 9A, 9B and 9C. FIG. 9A shows an obstacle detection system of a self-propelled type robot such as a cleaning robot. FIG. 9B shows a peripheral obstacle detection system of an automobile. FIG. 9C shows an application to an ultrasonic flowmeter.

In FIGS. 9A, 9B and 9C, the reference numerals 81, 81A, 81B, 81C, 81D and 81E show ultrasonic transmitter-receivers capable of performing transmission and reception of an ultrasonic wave almost in the horizontal direction according to the seventh embodiment of the present invention described in particular connection with any one of the first embodiment, the second embodiment, the fourth embodiment, the fifth embodiment, and the sixth embodiment. Reference numerals 82, 82A, 82B, 82C, 82D and 82E denote directional pattern regions of ultrasonic waves transmitted and received by the ultrasonic transmitter-receivers 81, 81A, 81B, 81C, 81D and 81E, 83 denotes a self-propelled type robot, 84A, 84B, 84C, 84D and 84E denote obstacles, 85 denotes an automobile, and 86 denotes a measurement channel.

Referring to FIG. 9A, in the ultrasonic transmitter-receiver 81A, a surface of the casing section is arranged almost flush with the second surface region 8 in the vicinity of the top of the casing section of the self-propelled type robot 83. According to the ultrasonic transmitter-receiver 81A of the seventh embodiment of the present invention, the directional pattern region 82A of ultrasonic transmission and reception waves can be made almost horizontal (parallel) to a floor surface 88A and is able to perform scanning in the circumferential direction as illustrated in FIG. 9A. Therefore, the obstacles in all circumferential directions of the self-propelled type robot 83 can be detected without arranging ultrasonic transmitter-receivers 81A dispersedly around the casing section or providing the casing section with a special mechanical system for directing the ultrasonic transmitter-receiver 81A toward the periphery.

In FIG. 9B, the ultrasonic transmitter-receivers 81D, 81E, 81B and 81C are arranged almost flush with the lower surface of bumper sections at the front and rear ends of the body of the automobile 85 and with the wall surfaces of portions near the front and rear ends of the roof of the body without providing any special protrusion. Similarly to FIG. 9A, the directional pattern regions 82D, 82E, 82B and 82C of the ultrasonic waves transmitted and received can be made almost horizontal (parallel) to the ground surface 88B. Therefore, as shown in FIG. 9B, it becomes possible to detect obstacles 84D and 84E on the ground existing in blind spots in front of and behind the automobile 85, obstacles 84B and 84C of the roof such as a guardrail and a signboard and so on. Moreover, it is possible to particularly provide the roof portion with no protruding portion or the like for the ultrasonic transmitter-receivers, and accordingly, there is no possibility of impairing the degree of freedom of design and so on.

In FIG. 9C, the ultrasonic transmitter-receiver 81 is arranged in flush with the inner wall surface of the measurement channel 86. The directional pattern region 82 of the ultrasonic wave transmitted and received is almost parallel to the channel, and transmission and reception of an ultrasonic wave are performed with the opposite ultrasonic transmitter-receiver 81. In this case, there is no need to provide a concave portion or a convex portion in the normal channel since the ultrasonic transmitter-receiver 81 is not required to face directly, and highly accurate flow rate measurement is possible with the mobile state of the fluid steadily maintained in the channel. This will be described in detail below as the eighth embodiment. There are provided a transmitter circuit 701 for driving the ultrasonic transducer 81, a receiver circuit 702 for executing amplification, band limiting and so on of the ultrasonic wave received by the other ultrasonic transducer 81, a changeover circuit 703 for changing the direction of transmission and reception, a time measuring section 704 for measuring a propagation time on the basis of an output from the receiver circuit 702, a flow rate calculating section 705 for obtaining a flow rate on the basis of an output value from the time measuring section 704, a display section 706 for displaying the flow rate calculated in the flow rate calculating section 705 and so on, and a control section 707 for controlling the measurement timing and so on. Therefore, by transmitting an ultrasonic wave from one ultrasonic transducer 81 to the other ultrasonic transmitter-receiver 81 and receiving the ultrasonic wave that has passed through the fluid to be measured such as a gas by the other ultrasonic transmitter-receiver 81, the propagation time between the ultrasonic transmitter-receivers 81 and 81 is measured by the time measuring section 704. Subsequently, conversely by transmitting an ultrasonic wave from the other ultrasonic transducer 81 to the one ultrasonic transducer 81 and receiving the ultrasonic wave that has passed through the fluid to be measured such as a gas by the one ultrasonic transmitter-receiver 81, the propagation time between the ultrasonic transmitter-receivers 81 and 81 is measured by the time measuring section 704. As described above, the propagation time of the ultrasonic wave between the pair of ultrasonic transducers 81 and 81 is measured a prescribed number of times, and the flow rate of the fluid to be measured such as a gas is calculated by the flow rate calculating section 705 on the basis of the value in the flow rate calculating section 705. Therefore, the ultrasonic transducers 81 and 81 can perform transmission and reception. In this case, a flow rate calculation system is constituted of the elements from the transmitter circuit 701 to the control section 707.

Eighth Embodiment

An ultrasonic flowmeter for measuring the flow rate of a fluid by an ultrasonic wave, which is one example of the application equipment of the ultrasonic transmitter-receiver as one example of the ultrasonic sensor according to the eighth embodiment of the present invention will be described with reference to FIGS. 12A through 17.

Before describing the ultrasonic flowmeter of the eighth embodiment, reference is made to a conventional ultrasonic flowmeter.

In recent years, an ultrasonic flowmeter, which measures the mobile velocity of a fluid by measuring a time during which an ultrasonic wave is transmitted through a prescribed propagation path and measures a flow rate from the measured value, is being utilized for a gas meter, chemical reaction controlling, and so on.

The principle of measurement of the conventional ultrasonic flowmeter will be described below with reference to FIG. 17. In the ultrasonic flowmeter shown in FIG. 17, a fluid in a pipe is flowing at a velocity V in the direction of arrow V in FIG. 17. A pair of ultrasonic transmitter-receivers 401 and 402 are arranged oppositely to each other on pipe walls 403 of the ultrasonic flowmeter. Each of the ultrasonic transmitter-receivers 401 and 402 is provided with a transducing device (transducer) for converting electrical energy into mechanical energy and converting mechanical energy into electrical energy. This transducing device is constructed of, for example, a piezoelectric transducer of piezoelectric ceramics or the like and exhibits a resonance characteristic similarly to a piezoelectric buzzer and a piezoelectric oscillator.

Operation of the ultrasonic flowmeter will be described first in the case where the ultrasonic transmitter-receiver 401 is used as a transmitter of an ultrasonic wave and the ultrasonic transmitter-receiver 402 is used as a receiver of an ultrasonic wave.

If an AC voltage having a frequency in the vicinity of the resonance frequency of the ultrasonic transmitter-receiver 401 is applied to the piezoelectric transducer of the ultrasonic transmitter-receiver 401, then the ultrasonic transmitter-receiver 401 radiates an ultrasonic wave into the fluid in the pipe. This ultrasonic wave propagates along a propagation path L1 and reaches the ultrasonic transmitter-receiver 402. The piezoelectric transducer of the ultrasonic transmitter-receiver 402 receives this ultrasonic wave and outputs a voltage signal.

Subsequently, the ultrasonic transmitter-receiver 402 is operated as a transmitter of an ultrasonic wave. In concrete, by applying an AC voltage having a frequency in the vicinity of the resonance frequency of the ultrasonic transmitter-receiver 402 to the piezoelectric transducer of the ultrasonic transmitter-receiver 402, the ultrasonic transmitter-receiver 402 radiates an ultrasonic wave into the fluid in the pipe. The ultrasonic wave propagates along a propagation path L2 and reaches the ultrasonic transmitter-receiver 401. The piezoelectric transducer of the ultrasonic transmitter-receiver 401 receives this ultrasonic wave and outputs a voltage signal.

As described above, the ultrasonic transmitter-receivers 401 and 402, which are each one ultrasonic transducer, can produce the function of a receiver and the function of a transmitter. According to this ultrasonic flowmeter, ultrasonic waves are continuously radiated from the ultrasonic transmitter-receiver when an ac voltage is continuously applied and it becomes difficult to measure the propagation time. Therefore, a burst voltage signal having a carrier of a pulse signal is normally used as a drive voltage.

If an ultrasonic burst signal is radiated from the ultrasonic transmitter-receiver 401 by applying a drive burst voltage signal to the ultrasonic transmitter-receiver 401, then this ultrasonic wave burst signal propagates through the propagation path L1 of a distance L and reaches the ultrasonic transmitter-receiver 402 after a lapse of time t.

The ultrasonic transmitter-receiver 402 can convert only the ultrasonic wave burst signal that has propagated into an electric burst signal at a high signal-to-noise ratio. An ultrasonic wave burst signal is radiated by using this electric burst signal as a trigger and applying again the drive burst voltage signal to the ultrasonic transmitter-receiver 401.

A device as described above is called the “sing-around device”. Moreover, a time required for an ultrasonic pulse to reach the ultrasonic transmitter-receiver 402 from the ultrasonic transmitter-receiver 401 is called the “sing-around period”, and its reciprocal is called the “sing-around frequency”.

It is assumed that the flow velocity of the fluid flowing in the pipe is V, the velocity of the ultrasonic wave in the fluid is C and the angle between the flow direction of the fluid, and the propagation direction of the ultrasonic pulse is θ in the ultrasonic flowmeter of FIG. 17. Further, it is assumed that the time required for the ultrasonic pulse emitted from the ultrasonic transmitter-receiver 401 to reach the ultrasonic transmitter-receiver 402 (sing-around period) is t₁ and the sing-around frequency is f₁ when the ultrasonic transmitter-receiver 401 is used as a transmitter and the ultrasonic transmitter-receiver 402 is used as a receiver. At this time, the following expression (4) holds. f ₁=1/t ₁=(C+V cos θ)/L  (4)

Conversely, if it is assumed that the sing-around period is t₂ and the sing-around frequency is f₂ when the ultrasonic transmitter-receiver 402 is used as a transmitter and the ultrasonic transmitter-receiver 401 is used as a receiver, then the relation of following expression (5) holds. f ₂=1/t ₂=(C−V cos θ)/L  (5)

A frequency difference Δf between both the sing-around frequencies is expressed by the following expression (6) based on the expression (4) and the expression (5). Δf=f ₁ −f ₂=2V cos θ/L  (6)

As is apparent from the expression (6), the flow velocity V of the fluid can be obtained from the distance L of the propagation path of the ultrasonic wave and the frequency difference Δf. Then, the flow rate can be determined from the flow velocity V.

In the ultrasonic flowmeter of FIG. 17, a matching layer (not shown) is provided on the surface of transmission and reception of an ultrasonic wave in the piezoelectric transducer of the ultrasonic transmitter-receiver. This is intended to alleviate the difference in the inherent acoustic impedance (hereinafter referred to as an “acoustic impedance”) between the fluid to be measured and the piezoelectric element by a layer (matching layer) that has an intermediate acoustic impedance, and to suppress the reflection of the ultrasonic wave at the interface between media that have different acoustic impedances. There occurs an inconvenience that the ultrasonic wave emitted from the ultrasonic transmitter-receiver does not sufficiently enter the fluid to be measured when an interface of a large acoustic impedance difference exists in the propagation path of the ultrasonic wave, and this disadvantageously leads to impossible flow rate measurement or significantly reduced measurement accuracy. Therefore, in order to avoid the above inconvenience and improve the measurement accuracy of the ultrasonic flowmeter, it becomes important to appropriately set the acoustic impedance of the matching layer. The acoustic impedance is generally defined by the following expression (7). Acoustic Impedance=(Density)×(Acoustic Velocity)  (7)

The acoustic impedance of the piezoelectric transducer that generates ultrasonic vibrations is, for example, about 30×10⁶ kg·m⁻²·s⁻¹, and the acoustic impedance of air is about 400 kg·m⁻²·s⁻¹. When measuring the flow velocity of air, it is preferable to set the acoustic impedance of the matching layer to about 0.11×10⁶ kg·m⁻²·s⁻¹.

Conventionally, in order to form a matching layer having acoustic impedance intermediate between those of the piezoelectric transducer and air, a material obtained by solidifying a material (for example, glass balloons or plastic balloons) of a comparatively small density with a resin has been used.

However, there has been a problem that, even with the matching layer employed as described above, a propagation loss has inevitably occurred by all means and the measurement sensitivity has been reduced when propagating an ultrasonic wave from the piezoelectric transducer into a gas of air or the like. The reason why it is difficult to efficiently propagate an ultrasonic wave from a solid to a gas is that the acoustic impedance of the gas is significantly smaller than the acoustic impedance of the solid, and a strong reflection of the ultrasonic wave occurs at the interface even if the matching layer is interposed.

Moreover, in the ultrasonic flowmeter of the type shown in FIG. 17, a cavity portion for arranging an ultrasonic transmitter-receiver is necessary inside the channel of the flow measurement section, and the existence of this cavity portion sometimes cause the disorder of the flow of the fluid to be measured. Moreover, since the flow rate itself comes to have an extremely small quantity, the channel is required to be made minute for microchemical analysis. In the above case, the conventional construction has had a problem that the ultrasonic transmitter-receiver has been unable to be arranged in the channel and unable to be applied to the flow measurement of a minimum quantity.

The eighth embodiment of the present invention is made in view of the aforementioned problems and has the object of providing a highly sensitive ultrasonic flowmeter capable of also coping with the flow measurement of a ultrasmall quantity without disordering the flow inside the channel that is the flow measurement section.

The ultrasonic flowmeter of the eighth embodiment of the present invention includes a flow measurement section having an inner wall that defines the channel of the fluid to be measured, at least one ultrasonic transducer that is provided outside the channel space enclosed by the inner wall of the flow measurement section and performs transmission and/or reception of an ultrasonic wave, and an ultrasonic flowmeter that is arranged between the ultrasonic transducer and the channel space and provided with a propagation medium portion that forms the propagation path of the ultrasonic wave. The density ρ₁ of the propagation medium portion, the acoustic velocity C₁ of the propagation medium portion, the density ρ₂ of the fluid to be measured, and the acoustic velocity C₂ in the fluid to be measured are constructed so as to satisfy the relation expressed as (ρ₂/ρ₁)<(C₁/C₂)<1.

In a preferred embodiment, the number of the ultrasonic transducers is plural, a first ultrasonic transducer among the plurality of ultrasonic transducers is arranged so as to emit an ultrasonic wave to a second ultrasonic transducer among the plurality of ultrasonic transducers, and the second ultrasonic transducer is arranged so as to emit an ultrasonic wave to the first ultrasonic transducer.

In a preferred embodiment, the propagation medium portion has a first surface region that faces the ultrasonic vibration surface of the ultrasonic transducer and a second surface region that faces the channel space, and the second surface region of the propagation medium portion is inclined with respect to the first surface region.

In a preferred embodiment, the first surface region of the propagation medium portion is inclined in the direction of flow velocity of the fluid to be measured in the channel space, and the second surface region is parallel to the direction of flow velocity of the fluid to be measured in the channel space.

In a preferred embodiment, the second surface region of the propagation medium portion forms substantially no difference in level between it and the inner wall of the flow measurement section.

In a preferred embodiment, the density ρ₁ of the propagation medium portion, the incident angle θ₁ of an ultrasonic wave to the interface between the propagation medium portion and the fluid to be measured, the density ρ₂ of the fluid to be measured, and the approach angle θ₂ of the ultrasonic wave from the interface to the fluid to be measured are constructed so as to almost satisfy the relation expressed as ρ₂/ρ₁=cotθ₂/cotθ₁.

In a preferred embodiment, the fluid to be measured is a gas whose density ρ₂ is not greater than 10 kg·m⁻³.

In a preferred embodiment, the propagation medium portion is formed of a dry gel of an inorganic oxide or an organic polymer.

In a preferred embodiment, the solid frame portion of the dry gel is made hydrophobic.

In a preferred embodiment, the dry gel has a density of not greater than 500 kg/m³, and the dry gel has a mean pore diameter of not greater than 100 nm.

In a preferred embodiment, there is possessed a matching layer, which is provided between the ultrasonic transducer and the propagation medium portion and acoustically matches the ultrasonic transducer with the propagation medium portion.

In a preferred embodiment, the size of the channel space in the flow measurement section, measured in a direction perpendicular to the direction of flow velocity of the fluid to be measured, is not greater than one half of the wavelength at the center frequency of the ultrasonic wave in the fluid to be measured.

In a preferred embodiment, the ultrasonic transducer is constructed so as to form a convergence sound field.

In a preferred embodiment, the first surface region of the propagation medium portion is curved so as to form a lens surface.

The ultrasonic flowmeter of the present invention includes: a flow measurement section having an inner wall that defines a gas channel; a pair of ultrasonic transducers provided outside the channel space enclosed by the inner wall of the flow measurement section, for performing transmission and/or reception of an ultrasonic wave; and a pair of propagation medium portions arranged between each of the one pair of ultrasonic transducers and the channel space, for refracting the propagation path of the ultrasonic wave. The propagation medium portion has a first surface region that faces the ultrasonic vibration surface of the ultrasonic transducer and a second surface region that faces the channel space. The first surface region of the propagation medium portion is inclined with respect to the direction of flow velocity of the gas in the channel space, and the second surface region is almost parallel to the direction of flow velocity of the gas in the channel space.

The present inventors have found the fact that an ultrasonic wave can be propagated from a solid to a fluid (particularly, gas) causing almost no loss at the interface if the ultrasonic wave is appropriately refracted by using a propagation medium portion made of an appropriate material in an ultrasonic transmitter-receiver, and come to consider the present invention.

In the ultrasonic transmitter-receiver according to the eighth embodiment of the present invention, the propagation medium portion, which has the surface (first surface region) inclined in the flow direction of the fluid to be measured and the surface (second surface region) almost parallel to the flow direction of the fluid to be measured, is arranged between the ultrasonic transducer and the fluid to be measured. The second surface region of the propagation medium portion is matched with the plane that defines the channel of the fluid so as not to disorder the flow of the fluid.

The eighth embodiment of the present invention will be described below with reference to the drawings.

Reference is first made to the ultrasonic flowmeter of the eighth embodiment of the present invention with reference to FIGS. 12A and 12B. FIG. 12A shows the cross section along the lengthwise direction of an ultrasonic flowmeter 310 of the eighth embodiment. FIG. 12B shows the cross section taken along the line B-B of FIG. 12A perpendicularly to the lengthwise direction of the ultrasonic flowmeter 310.

The illustrated ultrasonic flowmeter 310 includes: a tubular flow measurement section 304 having an inner wall 340 that defines a channel of the fluid to be measured; a pair of ultrasonic transmitter-receivers (ultrasonic transducers) 301 a and 301 b provided outside the channel space 309 enclosed by the inner wall 340 of the flow measurement section 304, for performing transmission and/or reception of an ultrasonic wave; and propagation medium portions 303 a and 303 b arranged between the ultrasonic transmitter-receivers 301 a and 301 b and the channel space 309, for forming a propagation path of an ultrasonic wave. The fluid to be measured is assumed to flow in the direction of arrow 305 inside the channel space 309 enclosed by the inner wall 340 of the flow measurement section 304. The ultrasonic transmitter-receivers (ultrasonic transducers) 301 a and 301 b are any of the ultrasonic transmitter-receivers of the first through sixth embodiments. The propagation medium portions 303 a and 303 b correspond to the propagation medium portions 6, 6A, 6B, 6C, 6D, 6E, 6F, and 6G. It is to be noted that the casing is not shown for the sake of simplicity.

In the eighth embodiment, the ultrasonic wave radiation surface of the ultrasonic transmitter-receiver 301 a is inclined in the flow direction 305 of the fluid to be measured, and the ultrasonic wave emitted from the ultrasonic transmitter-receiver 301 a is diagonally incident on the inner wall of the flow measurement section 304. Then, the ultrasonic wave is refracted at the interface between the propagation medium portion 303 a and the fluid to be measured, and received by one ultrasonic transmitter-receiver 301 b through a propagation path 306.

The section (cross section perpendicular to the flow direction 305) of the channel space 309 in the eighth embodiment is rectangular as shown in FIG. 12B. The flow measurement section 304 of the eighth embodiment is produced by solidifying the components 304 a and 304 b with a sealing material 304 c. It is to be noted that the shape of the channel space 306 is not limited to the illustrated one and may be another shape (for example, circle).

The propagation medium portions 303 a and 303 b have a first surface region 331 that faces the ultrasonic vibration surfaces of the ultrasonic transmitter-receivers 301 a and 301 b and a second surface region 332 that faces the channel space 309. In the eighth embodiment, assuming that the density of the propagation medium portions 303 a and 303 b is ρ₁, the acoustic velocity of the propagation medium portions 303 a and 303 b is C₁, the density of the fluid to be measured is ρ₂, and the acoustic velocity of the fluid to be measured is C₂, then the material of the propagation medium portions 303 a and 303 b is selected so that the relation expressed by the following expression (8) is satisfied. (ρ₂/ρ₁)<(C _(l) /C ₂)<1  (8)

When measuring the flow rate of a gas, it is difficult to find the material that satisfies the aforementioned condition. The reason is that there are few solid materials whose acoustic velocity C₁ is smaller than the acoustic velocity C₂ of a gas. In order to provide the propagation medium portions 303 a and 303 b that satisfy the aforementioned condition in the eighth embodiment, the propagation medium portions 303 a and 303 b are formed of a dry gel of an inorganic oxide or a dry gel of an organic polymer. The solid frame portion of the dry gel employed in the eighth embodiment is made hydrophobic, and its density is not greater than 500 kg/m³. This dry gel is a nano-porous dry gel (nanoporous dry gel) having a mean pore diameter of not greater than 100 nm.

The solid frame portion of the dry gel of the inorganic oxide preferably has an ingredient of at least silicon oxide (silica) or aluminum oxide (alumina). Moreover, the solid frame portion of the dry gel of the organic polymer can be constructed of a general thermosetting resin or a thermoplastic resin. For example, there can be used polyurethane, polyurea, phenol cured resin, polyacrylamide, polymethyl methacrylate, or the like.

In the case where the propagation medium portions 303 a and 303 b are formed of a nanoporous dry gel that has a main ingredient of, for example, silica, if the density ρ₁ is 200 kg/m³, then the acoustic velocity C₁ can be set within a range of about 100 m/s to 180 m/s. When the fluid to be measured is air, since the density ρ₂ of air is 1.22 kg/m³ and the acoustic velocity C₂ is 340 m/s, it is possible to concurrently satisfy the relations expressed as ρ₂<ρ₁ and C₁<C₂ and satisfy the relation expressed as (ρ₂/ρ₁)<(C₁/C₂) by adopting the propagation medium portions 303 a and 303 b. When measuring the flow rate of a gas such as natural gas, the propagation medium portions 303 a and 303 b preferably has a density ρ₁ ranging from 100 to 300 kg/m³ and an acoustic velocity C₁ ranging from 100 to 300 m/s.

The ultrasonic transmitter-receivers 301 a and 301 b have a piezoelectric element that functions as an ultrasonic transducer, and the transmission and/or reception of an ultrasonic wave can be performed. As a piezoelectric element, piezoelectric ceramics are suitably employed.

In the ultrasonic flow measuring unit 310 of the eighth embodiment, a matching layer 302 a is provided between the propagation medium portion 303 a and the ultrasonic transmitter-receiver 301 a, and a matching layer 302 b is provided between a propagation medium portion 303 b and an ultrasonic transmitter-receiver 301 b. The matching layers 302 a and 302 b have functions to improve the acoustic matching between piezoelectric ceramics (acoustic impedance: 30×10⁶ kg·m⁻²·s⁻¹) that are the ultrasonic generation sources of the ultrasonic transmitter-receivers 301 a and 301 b and the propagation medium portions 303 a and 303 b.

When forming the propagation medium portions 303 a and 303 b of the nanoporous dry gel (acoustic impedance: 3×10⁴ kg·m⁻²·s⁻¹) having a main ingredient of silica, by adopting matching layers 302 a and 302 b produced from a material of an acoustic impedance in the vicinity of 1×10⁶ kg·m⁻²·s⁻¹, the propagation efficiency of ultrasonic energy can be made to be almost one and more concretely be not smaller than 0.95. The material as described above can be provided by a composite material obtained by solidifying hollow glass balls with a resin material. The thickness of the matching layers 302 a and 302 b is preferably set to a quarter wavelength of the ultrasonic wave used.

Next, the behavior of an ultrasonic wave propagating from the propagation medium portion 303 a to the fluid to be measured will be described in detail below with reference to FIG. 13.

According to the aforementioned relation of arrangement, an ultrasonic wave enters an interface S along a direction inclined from the normal direction of the interface S between the propagation medium portion 303 a and the fluid to be measured. It is assumed that the incident angle of the ultrasonic wave with respect to the normal direction with the interface is θ₁ (0°<θ₁<90°). At this time, the ultrasonic wave is refracted at the interface S between the propagation medium portion 303 a and the fluid to be measured and enters the fluid to be measured at an angle (approach angle) θ₂ with respect to the normal direction of the interface S (θ₁<θ₂).

In the eighth embodiment, when ρ₂ of the fluid to be measured is given, various parameters (ρ₁, θ₁, and θ₂) are set so as to approximately satisfy the relation of the following expression (9). (ρ₂/ρ₁)=(cotθ ₂ /cotθ ₁)  (9)

With the above setting, the propagation efficiency of ultrasonic energy from the propagation medium portion 303 a to the fluid to be measured becomes almost one. At this time, the incident angle θ₁ satisfies the condition expressed by the following expression (10). (cotθ ₁)²=[(c ₁ /c ₂)²−1]/[(ρ₂/ρ₁)²−(c ₁ /c ₂)²]  (10)

Therefore, if ρ₁ and C₁ of the propagation medium portion 303 a and ρ₂ and C₂ of the fluid to be measured are determined, then the incident angle θ₁ is determined according to the expression (10). Moreover, if the incident angle θ₁ is determined, then the approach angle θ₂ is determined according to the expression (9).

If the incident angle θ₁ and the approach angle θ₂ are determined, then the inclination angle of the first surface region 331 of the propagation medium portion 303 a and the interval between the two ultrasonic transmitter-receivers 301 a and 301 b and so on can also be determined.

The aforementioned fact is applied as it is when the ultrasonic wave is received.

In the eighth embodiment, by forming the propagation medium portions 303 a and 303 b of the aforementioned material, the acoustic velocity C₁ of the propagation medium portions 303 a and 303 b can be set to 180 m/s, and the density ρ₁ can be set to 200 kg/m³. When measuring the flow rate of air, the density ρ₂ of the fluid (air) to be measured is 1.22 kg/m³, and the acoustic velocity C₂ is 340 m/s. Therefore, it is proper to set the incident angle θ₁ to 32° and set the approach angle θ₂ to 89° according to the relations of the expression (9) and the expression (10). The approach angle θ₂ is close to 90°, and therefore, the ultrasonic wave in the fluid to be measured is to propagate in a direction almost parallel to the flow direction 305.

A size H (see FIG. 12A) of the channel space 309 enclosed by the inner wall 340 of the flow measurement section 304 in the eighth embodiment is preferably set to a wavelength of not greater than a half wavelength or idealistically not greater than a quarter wavelength of the ultrasonic wave in the fluid to be measured. By setting the size of the channel space 309 to the size as described above, the appearance of a propagation mode due to the acoustic wave reflection in the channel space 309 can be restrained, and the time measurement accuracy can be increased. For example, when the wavelength λ of the ultrasonic wave to be used is about 4 mm, the size H of the channel space 309 can be set to about 2 mm. In this case, assuming that the lowest flow velocity to be measured is 1 mm/s and the measurement accuracy of the propagation time is 1 ns (nanosecond), then the interval in the transverse direction between the ultrasonic transmitter-receivers 301 a and 301 b can be set to about 120 mm.

According to the eighth embodiment, almost no propagation loss occurs at the interface S between the propagation medium portions 303 a and 303 b and the fluid to be measured, and therefore, it is not required to match the acoustic impedances of them at this interface S.

The propagation medium portions 303 a and 303 b are not required to be constructed of a material whose density ρ₁ and acoustic velocity C₁ are thoroughly uniform but allowed to have a laminate structure in which a plurality of kinds of material layers of varied density ρ₁ and acoustic velocity C₁ are laminated. When the laminate structure as described above is possessed, although it is sometimes the case where an ultrasonic wave does not travel straightly in the propagation medium portions 303 a and 303 b, there is no special problem. An important point is that the density ρ₁, the acoustic velocity C, and the incident angle θ₁ of the propagation medium portions 303 a and 303 b are set so as to satisfy the aforementioned expressions in the region in the vicinity of the interface between the propagation medium portions 303 a and 303 b and the fluid to be measured.

Moreover, as shown in FIG. 12A, the ultrasonic flow measuring unit 310 includes: a transmitter circuit 701 for driving an ultrasonic transducer 81; a receiver circuit 702 for executing amplification, band limiting, and so on of the ultrasonic wave received by the other ultrasonic transmitter-receiver 81; a changeover circuit 703 for changing the direction of transmission and reception; a time measuring section 704 for measuring a propagation time on the basis of an output from the receiver circuit 702; a flow rate calculating section 705 for calculating a flow rate on the basis of an output value from the time measuring section 704; a display section 706 for displaying the flow rate and so on calculated in the flow rate calculating section 705; and a control section 707 for controlling the measurement timing and so on. Therefore, by transmitting an ultrasonic wave from one ultrasonic transducer 301 a or 301 b to the other ultrasonic transmitter-receiver 301 b or 301 a and receiving the ultrasonic wave that has passed through the fluid to be measured such as a gas by the other ultrasonic transmitter-receiver 301 b, the propagation time between the ultrasonic transducers 301 a and 301 b is measured by the time measuring section 704. Next, by transmitting an ultrasonic wave from the other ultrasonic transducer 301 b to the one ultrasonic transducer 301 a and receiving the ultrasonic wave that has passed through the fluid to be measured such as a gas by the one ultrasonic transducer 301 a or 301 b, the propagation time between the ultrasonic transducers 301 a and 301 b is measured by the time measuring section 704. As described above, the propagation time of an ultrasonic wave between the pair of ultrasonic transducers 301 a and 301 b is measured a prescribed number of times, and the flow rate of the fluid to be measured such as a gas is calculated by the flow rate calculating section 705 on the basis of the value in the flow rate calculating section 705. Therefore, the ultrasonic transducers 301 a and 301 b can perform transmission and reception. In this case, a flow rate calculation system is constituted of the elements from the transmitter circuit 701 to the control section 707.

Operation of the ultrasonic flowmeter of the eighth embodiment will be described next.

First of all, an AC voltage having a frequency in the vicinity of the resonance frequency (for example, about 100 kHz to 1 MHz) is applied from the transmitter circuit 701 that concurrently serves as the drive circuit of FIG. 12B to the ultrasonic transmitter 301 a. By this operation, the ultrasonic transmitter-receiver 301 a radiates an ultrasonic wave on the condition of an efficiency of almost one to the propagation medium portion 303 a through the matching layer 302 a.

The acoustic wave, which has propagated through the medium portion 303 a, is refracted at the interface between the propagation medium portion 303 a and the channel space 309, radiated in the channel space 309 with an efficiency of almost one, and propagated through the inside of the fluid to be measured. Subsequently, the ultrasonic wave reaches the ultrasonic transmitter-receiver 301 b through the propagation medium portion 303 b and the matching layer 302 b provided on the opposite side. The ultrasonic transmitter-receiver 301 b converts the received ultrasonic wave into a voltage to generate a voltage signal (electric signal). The method for measuring the propagation time of the ultrasonic wave by the flow rate calculating section 705 on the basis of this electric signal and converting the flow velocity into the flow rate is similar to that of the prior art. A structural example of the drive circuit is described in Japanese Unexamined Patent Publication No. 2000-298045 and Japanese Unexamined Patent Publication No. 2000-298047.

According to the eighth embodiment, there are provided the propagation medium portions 303 a and 303 b that exhibit appropriate density ρ₁ and acoustic velocity C₁, and the ultrasonic wave is refracted at an appropriate angle. Therefore, the propagation loss at the interface of the substances can be made almost zero, and flow measurement can be achieved at a satisfactory signal-to-noise ratio. Then, according to the eighth embodiment, the flow rate of a gas (for example, hydrogen gas), of which the measurement has been extremely difficult by the conventional ultrasonic flowmeter, can easily be measured.

Furthermore, according to the eighth embodiment, there is existing neither large unevenness nor difference in level, which may cause a disorder of flow, inside the channel space 309 of the flow measurement section 304, and extremely stable flow measurement becomes possible. Moreover, since the ultrasonic transmitter-receivers are arranged outside the channel space 309, the size of the channel space 309 can be arbitrarily designed not depending on the sizes of the ultrasonic transmitter-receivers. As a result, the size of the channel space 309 is reduced to allow the flow measurement of an ultrasmall quantity to be performed.

Ninth Embodiment

An ultrasonic flowmeter as one example of the ultrasonic sensor according to the ninth embodiment of the present invention will be described with reference to FIG. 14. FIG. 14 shows the cross section along the lengthwise direction of an ultrasonic flowmeter 320 of the ninth embodiment. Like reference numerals are given to the common members of the ninth embodiment and the aforementioned eighth embodiment.

Hereinafter, characteristic points of the ultrasonic transmitter-receiver 320 of the ninth embodiment will be described below, and no description is provided for portions similar to those of the ultrasonic flowmeter 310 of the eighth embodiment.

In the ultrasonic flowmeter 320 of the ninth embodiment, the ultrasonic transmitter-receivers 301 a and 301 b are constructed so as to form a convergence sound field. In concrete, the first surface region of propagation medium portions 308 a and 308 b are curved so as to form a lens surface. With this arrangement, a matching layer 308 has a concave type surface on the measured fluid side. With the above construction, the ultrasonic wave transmitted from an ultrasonic transmitter-receiver 301 a is to converge inside a propagation medium portion 308 a. This convergence effect enables the transmission and reception of an ultrasonic wave with a larger sound pressure by means of an ultrasonic transducer of same identical capability, and therefore, the signal-to-noise ratio can be further improved.

In each of the eighth and ninth embodiments described above, a first surface region 331 of the propagation medium portion is inclined in the flow velocity direction 305 of the fluid to be measured in the channel space 309, and the second surface region 332 is parallel to the flow velocity direction 305 of the fluid to be measured in the channel space 309. However, the present invention is not limited to the above construction. For example, as shown in FIG. 15A, it is acceptable to adopt a construction in which the second surface region 332 of the propagation medium portions 303 a and 303 b is inclined in the flow velocity direction 305 of the fluid to be measured in the channel space 309. According to the above construction, the interval between the two ultrasonic transmitter-receivers can be reduced. However, in the construction of FIG. 15A, a difference in level is formed between the inner wall 340 of the flow measurement section 304 and the second surface region 332 of the propagation medium portions 303 a and 303 b. In order to reduce or eliminate this difference in level, as shown in, for example, FIGS. 15B and 15C, it is proper to form an inclined surface in a part of the inner wall 340 of the flow measurement section 304 and make the inclined surface match with the second surface region 332 of the propagation medium portions 303 a and 303 b.

The second surface region 332 of the propagation medium portions 303 a and 303 b preferably has no substantial difference in level between it and the inner wall 340 of the flow measurement section 304. However, when the disorder of the flow does not pose a major problem, there may exist a difference in level or unevenness as shown in FIGS. 16A and 16B.

In each of the aforementioned eighth and ninth embodiments, one pair of ultrasonic transmitter-receivers has a substantially identical construction, and an arrangement rotationally symmetrical at an angle of 180° is adopted. However, the present invention is not limited to the above construction. It is acceptable to apply the construction of the eighth and ninth embodiments to one of the one pair of ultrasonic transmitter-receivers and give a different construction (for example, the construction shown in FIGS. 15A through 15C) to the other ultrasonic transmitter-receiver. Moreover, in the eighth and ninth embodiments, not only the transmission but also the reception of an ultrasonic wave are performed by an identical ultrasonic transducer by employing an ultrasonic transducer as an ultrasonic transmitter-receiver. However, the eighth and ninth embodiments of the present invention are not limited to the above construction. It is acceptable to employ separate ultrasonic transducers for wave transmission and wave reception.

According to the eighth and ninth embodiments of the present invention, the loss in propagating an ultrasonic wave into the fluid to be measured can be reduced to almost zero, and therefore, the flow rate of various fluids including a gas can be measured with high sensitivity.

Moreover, according to the eighth and ninth embodiments of the present invention, there is no need to provide a difference in level or unevenness inside the channel, and therefore, the embodiments can also cope with the flow measurement of an extremely small quantity without disordering the flow of the fluid to be measured.

According to the present invention, there are provided the ultrasonic transducer for performing the transmission and/or reception of an ultrasonic wave and the propagation medium portion that forms the propagation path of the ultrasonic wave. By appropriately setting the mutual relation between the density ρ₁ and the acoustic velocity C₁ of the propagation medium portion and the density ρ₂ and the acoustic velocity C₂ of the fluid that stuffs the circumjacent space and appropriately refracting the ultrasonic wave at an appropriate angle, the loss in radiating an ultrasonic wave into the fluid that stuffs the circumjacent space can be reduced to almost zero, and/or the loss in receiving the ultrasonic wave entering from the fluid that stuffs the circumjacent space can be reduced to almost zero. Therefore, highly efficient wave transmission and reception become possible for various fluids including a gas. Moreover, when the fluid is a gas, the wave transmission and reception are can be performed almost horizontally (parallel) with respect to the wave transmission and reception surface of the ultrasonic transmitter-receiver, and the present invention can be used for developing a variety of applications.

By properly combining arbitrary embodiments of the aforementioned various embodiments, the effects possessed by them can be produced.

Although the present invention has been fully described in connection with the preferred embodiments thereof with reference to the accompanying drawings, it is to be noted that various changes and modifications are apparent to those skilled in the art. Such changes and modifications are to be understood as included within the scope of the present invention as defined by the appended claims unless they depart therefrom. 

1-29. (canceled)
 30. An ultrasonic sensor for performing transmission or reception of an ultrasonic wave to a circumjacent space stuffed with a fluid, the sensor comprising: an ultrasonic transducer; and a propagation medium portion that is stuffed in a space between the ultrasonic transducer and the circumjacent space, for forming a propagation path of the ultrasonic wave.
 31. An ultrasonic sensor for performing transmission or reception of an ultrasonic wave to a circumjacent space stuffed with a fluid, the sensor comprising: an ultrasonic transducer; and a propagation medium portion that is arranged between the ultrasonic transducer and the circumjacent space, for forming a propagation path of the ultrasonic wave, wherein a density ρ₁ of the propagation medium portion, an acoustic velocity C₁ in the propagation medium portion, a density ρ₂ of the fluid that stuffs the space, and a sound velocity C₂ in the fluid that stuffs the space satisfy a relation expressed as (ρ₂/ρ₁)<(C₁/C₂)<1.
 32. The ultrasonic sensor as claimed in claim 31, wherein the propagation medium portion has a first surface region that faces an ultrasonic vibration surface of the ultrasonic transducer and a second surface region that faces a flow that stuffs the circumjacent space, and the second surface region of the propagation medium portion is inclined with respect to the first surface region.
 33. An ultrasonic sensor for performing transmission or reception of an ultrasonic wave to a circumjacent space stuffed with a fluid, the sensor comprising: an ultrasonic transducer; a propagation medium portion that is arranged between the ultrasonic transducer and the circumjacent space, for forming a propagation path of the ultrasonic wave; and a reflector that is arranged in contact with the propagation medium portion, for controlling the propagation path of the ultrasonic wave, wherein a density ρ₁ of the propagation medium portion, an acoustic velocity C₁ in the propagation medium portion, a density ρ₂ of the fluid that stuffs the space, and a sound velocity C₂ in the fluid that stuffs the space satisfy a relation expressed as (ρ₂/ρ₁)<(C₁/C₂)<1.
 34. The ultrasonic sensor as claimed in claim 33, wherein the propagation medium portion has a first surface region that faces an ultrasonic vibration surface of the ultrasonic transducer, a second surface region that faces a flow that stuffs the circumjacent space and at least one third surface region that is arranged between the first surface region and the second surface region in the propagation path of the ultrasonic wave and brought in contact with the reflector, and the second surface region of the propagation medium portion is inclined with respect to at least one of the first surface region and the third surface region.
 35. The ultrasonic sensor as claimed in claim 30, wherein a density ρ₁ of the propagation medium portion, an incident angle θ₁ of an ultrasonic wave to an interface between the propagation medium portion and the fluid that stuffs the circumjacent space, a density ρ₂ of the fluid that stuffs the circumjacent space, and an approach angle θ₂ of the ultrasonic wave from the interface to the fluid that stuffs the circumjacent space almost satisfy a relation expressed as ρ₂/ρ₁=cotθ₂/cotθ₁.
 36. The ultrasonic sensor as claimed in claim 30, wherein the propagation medium portion is formed of a dry gel of an inorganic oxide or an organic polymer.
 37. The ultrasonic sensor as claimed in claim 35, wherein a solid frame portion of the dry gel is made hydrophobic.
 38. The ultrasonic sensor as claimed in claim 36, wherein a density of the dry gel is not greater than 500 kg/m³, and a mean pore diameter of the dry gel is not greater than 100 nm.
 39. The ultrasonic sensor as claimed in claim 30, comprising: an acoustic matching layer that is provided between the ultrasonic transducer and the propagation medium portion, for acoustically matching the ultrasonic transducer with the propagation medium portion.
 40. The ultrasonic sensor as claimed in claim 30, wherein the fluid that stuffs the circumjacent space is a gas having a density ρ₂ of not greater than 10 kg/m³.
 41. The ultrasonic sensor as claimed in claim 30, wherein a direction of transmission or reception of an ultrasonic wave is almost parallel to the second surface region.
 42. An ultrasonic flowmeter comprising: a flow measurement section having an inner wall that defines a channel of a fluid to be measured; at least one ultrasonic transducer that is provided outside a channel space enclosed by the inner wall of the flow measurement section, for performing transmission or reception of an ultrasonic wave; and a propagation medium portion that is arranged between the ultrasonic transducer and the channel space, for forming a propagation path of the ultrasonic wave, wherein a density ρ₁ of the propagation medium portion, an acoustic velocity C₁ in the propagation medium portion, a density ρ₂ of the fluid to be measured, and a sound velocity C₂ of the fluid to be measured satisfy a relation expressed as (ρ₂/ρ₁)<(C₁/C₂)<1.
 43. The ultrasonic flowmeter as claimed in claim 42, wherein a plurality of ultrasonic transducers are provided, a first ultrasonic transducer among the plurality of ultrasonic transducers is arranged so as to emit an ultrasonic wave to a second ultrasonic transducer of the plurality of ultrasonic transducers, and the second ultrasonic transducer is arranged so as to emit an ultrasonic wave to the first ultrasonic transducer.
 44. The ultrasonic flowmeter as claimed in claim 42, wherein the propagation medium portion has a first surface region that faces an ultrasonic vibration surface of the ultrasonic transducer and a second surface region that faces the channel space, and the second surface region of the propagation medium portion is inclined with respect to the first surface region.
 45. The ultrasonic flowmeter as claimed in claim 44, wherein the first surface region of the propagation medium portion is inclined in a direction of flow velocity of the fluid to be measured in the channel space, and the second surface region is parallel to the direction of flow velocity of the fluid to be measured in the channel space.
 46. The ultrasonic flowmeter as claimed in claim 44, wherein the second surface region of the propagation medium portion forms substantially no difference in level between the second surface region and the inner wall of the flow measurement section.
 47. The ultrasonic flowmeter as claimed in claim 42, wherein the density ρ₁ of the propagation medium portion, an incident angle θ₁ of an ultrasonic wave to an interface between the propagation medium portion and the fluid to be measured, the density ρ₂ of the fluid to be measured, and an approach angle θ₂ of the ultrasonic wave from the interface to the fluid to be measured almost satisfy a relation expressed as ρ₂/ρ₁=cotθ₂/cotθ₁.
 48. The ultrasonic flowmeter as claimed in claim 42, wherein the fluid to be measured is a gas having a density ρ₂ of not greater than 10 kg·m⁻³.
 49. The ultrasonic flowmeter as claimed in claim 42, wherein the propagation medium portion is formed of a dry gel of an inorganic oxide or an organic polymer.
 50. The ultrasonic flowmeter as claimed in claim 49, wherein a solid frame portion of the dry gel is made hydrophobic.
 51. The ultrasonic flowmeter as claimed in claim 50, wherein a density of the dry gel is not greater than 500 kg/m³, and a mean pore diameter of the dry gel is not greater than 100 nm.
 52. The ultrasonic flowmeter as claimed in claim 42, comprising: a matching layer that is provided between the ultrasonic transducer and the propagation medium portion, for acoustically matching the ultrasonic transducer with the propagation medium portion.
 53. The ultrasonic flowmeter as claimed in claim 42, wherein a size of a channel space in the flow measurement section, the size being measured in a direction perpendicular to a direction of flow velocity of the fluid to be measured, is not greater than a half wavelength of the ultrasonic wave at a center frequency in the fluid to be measured.
 54. The ultrasonic flowmeter as claimed in claim 42, wherein the ultrasonic transducer forms a convergence sound field.
 55. The ultrasonic flowmeter as claimed in claim 53, wherein the first surface region of the propagation medium portion is curved so as to form a lens surface.
 56. An ultrasonic flowmeter comprising: a flow measurement section having an inner wall that defines a channel of a gas; a pair of ultrasonic transducers that are provided outside a channel space enclosed by the inner wall of the flow measurement section, for performing transmission or reception of an ultrasonic wave; and a pair of propagation medium portions that are arranged between each of the one pair of ultrasonic transducers and the channel space, for refracting a propagation path of the ultrasonic wave, the propagation medium portion comprising a first surface region that faces an ultrasonic vibration surface of the ultrasonic transducer and a second surface region that faces the channel space, the first surface region of the propagation medium portion being inclined in a direction of flow velocity of the gas in the channel space, and the second surface region being almost parallel to the direction of flow velocity of the gas in the channel space.
 57. An apparatus comprising: the ultrasonic flowmeter claimed in claim 42; a pipe for supplying a fluid to be measured to the ultrasonic flowmeter; and a display section for displaying a flow rate measured by the ultrasonic flowmeter.
 58. An ultrasonic sensor for performing transmission or reception of an ultrasonic wave to a circumjacent space stuffed with a fluid, the sensor comprising: an ultrasonic transducer; and a propagation medium portion that is stuffed in a space between the ultrasonic transducer and the circumjacent space, for forming a propagation path of the ultrasonic wave, wherein the propagation medium portion has a first surface region that faces an ultrasonic vibration surface of the ultrasonic transducer and a second surface region that faces a flow stuffing the circumjacent space, and the second surface region of the propagation medium portion is inclined with respect to the first surface region. 